I can't really imagine a successful "loving" threesome with two guys and a girl in which the guys are at least a bit accepting of each other. One of the nicest warm-up positions in a threesome is when everybody lies in a triangle and sucks the next person's genitals.
And then there's the two on top position. One of the men lies on his back and rims the anus/sucks the pussy of the person sitting on his face, while the other person rides his dick, while kissing or fondling the breasts of the other person on top.
Of course there are other positions in which the girl is the centre of attention, but if you go in for double penetration it's - oh danger - quite likely that penises and testicles may touch!
Of course if the guys only have eyes for each other, a girl would definitely feel scorned, But a moderate dose of bi-action surely spices up the evening.