What is the name of the trio of young ponies trying to earn their cutie marks?

The trio of young ponies trying to earn their cutie marks is called the Cutie Mark Crusaders.

The original members are Apple Bloom, Sweetie Belle, and Scootaloo. They form their club after realizing that all three are “blank flanks”—young ponies who have not yet discovered the special talent represented by a cutie mark.

Their early adventures involve trying unusual activities, stunts, and schemes in hopes of finding their talents. Apple Bloom is Applejack’s younger sister, Sweetie Belle is Rarity’s younger sister, and Scootaloo admires Rainbow Dash while learning how to use her own strengths.

The trio eventually earns its cutie marks in the fifth-season episode “Crusaders of the Lost Mark.” Rather than ending the club, this milestone changes its purpose: the Crusaders begin helping other ponies understand their own talents. Babs Seed and Gabby later become honorary members, but the original group remains a trio.