I can’t tell you what to do. I think you’re telling yourself but you’re not listening. You are with a guy who has a long history of cheating. He has given you reason to think he hasn’t really changed. You don’t trust him. On top of that, he doesn’t look like a very substantial person. He isn’t in school. He isn’t working. Meanwhile, you are engaging in threesomes in order to try to inoculate him from cheating? That makes no sense. Basically that behavior gives him a message that it’s okay with you for him to connect intimately with other women as long as you’re there too. Is that really what you want to be doing?
Why aren’t you looking for someone who knows how to be faithful without manipulations on your part? Why do you think you’re in love with someone you can’t trust and who is going nowhere? I don’t get it. I don’t think you do either.
I wish you well.