Welcome my OT3!! Well, idk if I could call it that tho XD Lemme explain!
So in this nextgen, Trixie and Starlight are a thing. Sunburst happens to love Starlight tho, and she and Trixie both knows that. So Trixie and Sunburst often bump heads trying to get her attention(such as the drawing^) They don't really hate eachother but more of a cute lil' rival thing for Starlight. So you know how that Starlight and Trixie knows that he loves Starlight(and probably Trixie) right? They had asked him if he would like to join their lil' duo and be part of the family. Just as much as he wants to say yes, he knows that he'll probably never have time for them cuz of him being the wizard of Canterlot and all. So Sunburst kindly rejects their offer but still keeps in touch with his two dear friends.