You need to find your backbone. Your actual backbone. Reach around, feel it. It’s important, it keeps you standing up straight. Without it you’d be a puddle of unmovable goo. Notice how it curves. The gentle curve gives it strength, not weakens it. It’s when the backbone doesn’t curve, or curves in the wrong direction, that’s when it’s weak and can’t properly support your weight.
Learn from your backbone. It holds you up physically, your emotional health holds you up mentally. Your emotions need to curve too. But they need to curve in the right direction.
You agreeing to a threesome under duress is putting your backbone under strain, curving it the wrong way. Your boyfriend might seem respectful now, but how respectful do you think he’s going to stay if he feels like he can cheapen his relationship with you with someone else? Even if he’s not sleeping around now, how long do you think he’ll be able to keep himself from doing that while using you?
Your backbone needs to curve the right way. You don’t need him, you can handle the pain of being single again. It’s designed for that. You can focus on you without him getting in the way. Build you the way you want to be, not just accept what he wants you to be.
Relationships have to be built on respect, or they’re worse than useless. Don’t just tell him no. Tell him to fuck off with that nonsense. You don’t have to dump him, although honestly you probably should. Demand respect. You don’t even have to raise your voice. “No I’m not doing that, find another idea.”
You can try to be constructive. Shake things up yourself. But it sounds to me like he’s already losing respect for you. So he might not accept anything you try to do. This is how you know your relationship is over. When he’s only looking out for himself, not allowing you to be a participant. When he’s treating you like living room furniture to be rearranged and given away.