I fantasize about my best friend's girlfriend
That could lead to a tough situation, why do you fantasize about her?
The Girlfriend is quite hot and I like to see them together
So you are fantasizing about both of them and not just the girlfriend? Does your friend know? Are you hoping it will lead somewhere?
Threesome with my best friend and his hot girlfriend.
It seems like you are definitely attracted to both and don't plan on changing anything. Who knows, there could be a chance things could work out between all three of you!