Well, it's not so easy.
Let's say that the throuple is, indeed, the evolution of the Cassie-Eve relationship and so MC is really "the third wheel".
I think it's possible in real life a similar situation.
I've never seen the friendship between two males becoming a love relationship, but with my eyes I've seen the friendship between two girls becoming something "tighter" (love relationship or also just sex).
Well, I think that a man could get into a relationship like that and:
-be useful for the couple of girls, because approaching a male that both like, could help them to approach each other without feeling it as wrong (the two girls have lived through difficult years only thanks to each other, it's normal that they developed strong feelings);
-be useful for the man. Not only for the quite obvious lust and for the dream that all men have about threesome, but because I really guess that a relationship (deep, deeper or light) with two women could be very intense and difficult, but very satisfying and interesting too (not only for sex, but for feelings);
-be useful for all three, because all three could get something from the relationship.
And so the third wheel could become a wheel important as much as other two.
Kelly could be a very interesting path (she's completely a new girl, a girl who wants "to re-write herself"), but also the throuple could be a very interesting path. Demanding, uneasy to live into, but very interesting.
In real life I'd prefer a threesome where the two girls have feelings one for each other, rather than just lust one for each other or, worse, just fake lust.