To answer one of your questions: suggesting a threesome as a way to try and get over insecurity and jealousy is a really bad idea. That kind of sexual encounter can get messy and unpleasant very quickly if someone is doing it for the wrong reasons.

Let’s clarify what you already know, and have said yourself: your girlfriend didn’t do anything wrong. She was single, her sexual partners were consenting adults. It’s also pretty normal to remain friends with someone you once had a sexual relationship with, especially if you were friends beforehand. So, the question is… if you stay together, are you going to accept that and move forward, or are you going to seethe for years about how uncomfortable you are about it?

If you feel like you can’t handle the fact that she once slept with two of her friends, and is continuing to be friends with them? It’s probably not fair to either of you to continue a relationship with each other.