Many men are (or atleast seem to be) obsessed with 2 women in a relationship, maybe its similar to that. It doesn't make you anything, and there's nothing wrong with it. Don't worry about it, I'm not much older than you so I remember very clearly that 15 can be a very weird/confusing age. Now I feel old...lol

ANYWAY, don't worry about it. Not everyone feels the same way, but that doesn't mean its not normal.