Please don't do anything you'll regret later
I am assuming that you, like myself, are not a criminal and cannot afford a criminal record. A good number of murders that occur in North America are crimes of passion provoked by situations much like yours. A few moments of rage can change the path of your life forever.
The best thing to do would be just to confront them seperately.
Get straight to the point, and sever all ties as neatly as possible.
Don't try to guilt them or make them feel bad. They won't, and you'll just feel more miserable for wasting the energy and effort.
They are clearly pieces of ****e and don't deserve any further consideration.
I used to have a training partner that works as an enforcer (thug) for a local gang boss. He CAN, and DOES, get away with random acts of violence, because he has nothing to lose. He is living off the grid, has been to jail, and living on impulse is one of the few luxuries he has.
According to him, when he discovered one day that one of his friends was sleeping with his EX-girlfriend, he got a few guys together, kidnapped the "friend" from his own house, threw him into the trunk of his car, and brought him to the woods. They tied him up in a canvas sack and beat him (carefully), and left him in the forest tied to a tree until the next day.
Bro's before Ho's.
I'm actually angry hearing your story.
I've had a couple of dozen girlfriends in the past ten years, but only one best friend. I love my girlfriend, but I told her early on that if she's got a problem with my best friend, then we should end it right away, because it's not even a choice for me.