Thanks for explaining the situation better. At first it seemed to me like you had double standards (Drew didn't tell you about Brian right away, and presumably didn't tell him about you right away either, then he did the same thing with Michael, and you're upset about the second time while obviously you knew he was like that since that's how you met) but then you add that they asked you to break up with someone else because they wanted you to only date them, so it seems to me it made sense for you to expect the same thing in return.
Then you say they told you several times that it was going to just be the three of you, and then Drew lied about talking to a new person, and Brian apparently covered for Drew.
The fact that you are jealous is one thing, however I'm trying to put that aside and look at the whole situation. It seems to me Drew and Brian have been together for a while. Drew likes talking to other guys, and Brian doesn't like it too much, but seems to have tolerated it in the past, or be used to it, or be fine with it as long as the guy Drew likes is someone he (Brian) likes too.
When you entered the relationship, fidelity was expected, you say. But wasn't it also expected between Drew and Brian? It seems that Drew has a problem with that. We know of at least two occasions (you and Micheal) when he talked to someone else online and wasn't being honest. For that matter, do you know that Michael is aware of the situation? You weren't told about Brian right away, after all.
Then there is the matter of how you reacted. It was your first experience, and you felt strongly about things, and you dealt with them as you could at the time... the fact is that they found it controlling and manipulative, and well, from the way you describe it, it does seem like you wanted to get them to do things without going to talk to them. You wanted to provoke a reaction. You wanted to shock them, etc. You probably thought it would "teach them" and they would realise they could lose you, and change their behaviour?
Instead they thought "we don't need that kind of things" and aren't interested in a relationship with you anymore.
I think you need to see your own mistakes here. Admitting you reacted in wrong ways doesn't mean they were in the right. I can see why you would say Drew cheated if he said he wouldn't do it and still did, and lied about it. Then again, maybe he was just chatting with that guy and didn't plan on a serious relationship with him, and therefore didn't think of it as cheating. How clear were you about your boundaries?
So many things are "obvious" to people who are in a relationship that they don't talk about them. And then they realise it was also "obvious" to their partners, but not the same thing was obvious!
Maybe they expected YOU to be faithful to them, but never felt they had to do the same. Maybe Drew thought chatting would be fine. Maybe he's been doing that kind of things for years and you were the fluke that became an actual relationship rather than something on the side.
I don't know if you can start a relationship again with them, but if you want to you'll need to heal from the relationship first and put it all behind you.