so this is not with all three at once? you mentioned the other 3 girls, so is it a quasi-orgy or something? bed-swapping? what do you mean, exactly?
anyway, I don't know if it's necessarily immoral, there are a lot worse things you could do, but it just sounds like you're doing it for the sake of doing it, like it's some kind of initiation or rite of passage, and just remember that if you're all in the same circle of friends, and you do this with all 3 guys, there's probably not going to be much hope for you having any sort of quality relationship with these "hot, very smart" guys with "great jobs and promising futures"
the truth is, not many guys look to have serious relationships with girls that fuck all their friends in a wild, debauchery-type atmoshpere and setting... just something to think about, I know it seems like a double standard, that guys will be promiscuous, then expect to marry a "good girl," but that's the way it is, and they will be checking you off their lists, there isn't any doubt about that
and i also get the impression you feel pressured by your circle of friends, and are going through with this despite obvious doubts
I had a friend who did this sort of thing a few times, let me just say she ended up feeling very bad about herself and it destroyed her self esteem completely, she had been in a relationship that she thought was going to end in marriage, but her boyfriend ran off with someone and it precipitated this sort of behaviour and things got very bad, I don't really know anything about you and your history, but in most cases this sort of thing is a very bad idea