A GM's Guide To: G3

Pinkie Pie: A departure from G4, she's a much more levelheaded, though still lighthearted, character.  She's often considered an unofficial leader that's a bit of a problem solver and has a surprising amount of dry sarcasm in her humor style.  Her personal quirks are a love of pink, her infamous love of parties, and a subtle lisp though such a thing would only be noticeable in Skype and tabletop.  If fully statted out, she'd be generally even stats with Instant-Party and various teamwork/aiding utilities.  As the all rounder/leader for this generation, she intriguingly fits with almost every Element/Virtue with Laughter being the lowest priority.  G3 Pinkie Pie is a rather stable character who makes a large number of appearances and retains the same personality through the Core 7 mini reboot.
Rainbow Dash: Essentially, G4 Rarity.  Sophisticated and speaking in a faux British accent, she's well meaning despite sometimes being a little fussy.  In the beginning she comes off as just a tiny bit older than the rest of the cast.  The Core 7 mini reboot makes her less overtly "mature" since she's flanderized into a more generic fashionista and shopaholic but she instead winds up being the physically largest.  She also gets a more explicit job as a seamstess instead of a vague decorator who loves rainbows and is sometimes in charge of some festivals.  The original version tended to work "darling" into almost every sentence while the Core 7 version who occasionally use "how dashing!" at the end of a few sentences.  A relatively Precision and Horse-Sense oriented build, she'd have things like Fabricate and Freaky Knowledge.  She's oddly the Kindness candidate since she'll offer the olive branch but Generosity is also a very valid Element choice.
Minty: A well meaning but clumsy ditz. This tends to cause a love or hate response to her.  Can be an agent of change to a situation or simply a friendly, if off-kilter, NPC in the campaign.  Her character quirks include a love of the color green and a large sock collection.  Precision is her dump stat, Jinx! her primary utility talent, and her Element would be Laughter.
Spike: Instead of the good natured yet occasional snark from G1 and G4, here he's rather stuffy and rules oriented.  He's still good hearted and a wealth of information but winds up inadvertently antagonistic at times.  Actually has rather few appearances compared to the other two versions of Spike.  Never confirmed to have a breath attack but might actually be Acidborn in this generation since he was found in a rancid cave and had no problems with it.  High Knowledge with a focus on History and presumably Honesty as an Element.
Wysteria: A soft spoken gardener who's quite levelheaded and organized.  Initially just another part of the side cast, she eventually gets coronated as Ponyville's princess due to the events where they find Spike.  Can either be an official leader for a campaign's setting or merely another important NPC since she shows some difficulty adjusting to the role of princess due to the rules and restrictions until she decrees changes.  Loves flowers and is willing to get dirty while she hates weeds enough to fly into a rage.  Medium to high Knowledge with Applejack of All Trades, Green Hooves, and the Element of Generosity.
Sunny Daze: A laid back athlete with a penchant for enjoying the sun and a tendency to use short nicknames for friends.  Has a mild surfer vibe to her.  While never an important character, she's a rather actively recurring secondary that fills the athlete role until Scootaloo is introduced.  If made into a full character rather than named background filler, she'd have high Brawn with various utilities augmenting Athletics.

Combining the two since they have similar plots but go about it differently.  ACB is flawed since its has a lot of filler and less of an overt plot.  Worth noting that PP is half the length.  The simplicity allows them to be used as an introduction to the setting and characters for the campaign.  While G3 as a whole has less conflict than other generations, the better arcs do setup greater conflicts and a fair bit of worldbuilding.  Death is fully possible though the lack of outside dangers greatly lessens that likelihood.

As evidenced by one of the titles, a member of the community is having a birthday celebration.  While not heavily promoted by either story, the individual is someone of importance so that it actually makes sense for a sizable chunk of town to be involved with the celebration.  Alternate campaign ideas would be the anniversary of an event an NPC has been involved in such as some sort of prior award or perhaps saving the town.  

For ACB, the problem is that no one can decide how to handle the event, especially since that NPC is who they usually ask for help in solving disputes.  Multiple parts of the community are in friendly competition to show their appreciation.  This can be handled multiple ways, ranging from multiple skill checks involved in constructing a gift or decoration to various forms of persuasion among community members to get them to cooperate.  For a more combat oriented campaign, this can be the divergent point where adventuring can happen.  Either an object/ingredient is needed outside of town or something has been stolen and needs to be retrieved.  Afterwards, the NPC can be brought to the celebration at Ponyville Castle.

For PP, the town is in agreement of what to do and the primary concern was keeping the NPC away from the area as they prepare.  While construction/decoration is still an option, the primary distraction is setup in the form of a scavenger hunt.  This was handled mostly offscreen but for a RPG it can be as harmless or as dangerous as needed for the campaign. It can also serve as another worldbuilding activity.  Similarly, the end result is bringing the NPC to the celebration.

DitC is essentially two plots (and a lot of filler).  One is super short and easy to pull out while the other ties into the rest of the campaign's worldbuilding arc.  Like the above arc, PPatLBJ is a similar mirror that might as well be treated as the same for this.  An NPC needs to prepare a dance or musical performance and due to various reasons they have not been able to.  Both only seem to vaguely note time constraints but a campaign can more overtly give a reason such as the preparations from the prior arc, injuries from a prior mishap, or perhaps the performance was requested but the request wasn't given until the last moment.  Either way, crunch time.  From here, some sort of suggestion can be given for something the NPC and their performers can do.  PC participation allows for a skill challenge and better relations with the NPCs.

From a personal and subjective standpoint, A Charming Birthday and Dancing in the Clouds are among G3's weakest arcs in their original animated form due to massive filler and poor animation. They really feel more like portions of another session than a full session of their own. However, DitC does startup G3's worldbuilding arc which is decent, if subtle.

Sparkleworks: Best friend of Sunny Daze, she's the Precision counterpart to Sunny's Brawn.  She prefers dancing, glitter, and electrical lighting instead of raw athleicism and sunlight.  It's vaguely implied that she may be in charge of electrical utilities or at least that she would have a Freaky Knowledge for lighting and wiring.  She shows up early on but never gets a well defined personality.
Sweetberry: Another early revealed character that fills in the background but never got much character development except a passive-agressive argument with Rainbow Dash at one point and sometimes adding "Sweet!" as a descriptive term.  She acts as the resident baker until Sweetie Belle is introduced.  As is obvious, she has a love of berries and is in charge of rainbow berry storage.
Star Catcher: Leader of the pegasi, she's stated to explicitly have the ability to grant heartfelt wishes.  She's shown to be levelheaded and good with navigation.  For Skype or tabletop, her voice is either going to come off slightly regal or accidentally seductive.  Star Catcher also has a slightly larger physical build than the rest of the cast, though most of that is her wingspan.  She has magical talking butterflies that can serve as Mooks or a Companion pending on playstyle.
Thistle Whistle: With pegasus ponies being shy as a race in G3, Thistle serves as a grumpy dissenter to counterpart Star Catcher's open kindness.  She's known for periodically whistling in between her words.  She's also a pegasus who's afraid of clouds though there's never any backstory to why.  She has been shown to get over her fears if a friend is in danger.
The rest of the pegasi are just background filler.  At least one named character would be Coconut Grove as a regular pegasus who's more willing to listen to Star Catcher instead of Thistle Whistle's disagreements.

Unlike the other parts which were early stories and a counterpart that was similar while this is two stories that lead into one plot.  Through various means, a pony from Ponyville discovers the portal to Butterfly Island.  In DitC, the pony Skywishes makes a wish to help her friend come up with a special dance and Star Catcher heard the wish.  For a campaign, there's any number of ways the initial meeting with Star Catcher can be handled ranging from her being curious and flying through to the players getting flung into it due to a mishap.  Either way, the initial meeting is one of curiosity.  It's agreed that she and the players should meet again.  Upon the second interaction, Star Catcher feels the two sides should meet.  The story essentially becomes one of cultural interaction.  The Ponyville ponies are skeptical of flying ponies while the pegasi of Butterfly Island are xenophobic of outsiders.  From here, it comes down to the players and Star Catcher figuring out how to get the two groups to meet.  In FANFA, Star Catcher saves Pinkie Pie from a fall while the town brings the welcoming celebration to Butterfly Island.  Both can worth, using skill checks for both the interactions and the preparation.  The roles can also be reversed with someone saving a pegasus and that act of bravery being the push to get them to visit Ponyville.