Hiya, PMD, and welcome to the forum.
I'm really sorry for your pain.
My heart goes out to you and to your men.
The heart of successful poyamory is free and open communication between people who love each other. I'm glad to see the three of you have invested so much in establishing that. I know it can be hard work, but it's so important. So kudos to you on that front.
May I make an observation? Your boyfriend, SL, does not share your polyamorous mindset. For many people, that by itself would make a successful relationship a non-starter. (There are, of course, very happy and successful exceptions, such as MonoV and Redpepper and her husband.)
Only the individuals involved can make the decision as to what they can and cannot accommodate in thier lives and hearts. Sadly, SL has now stated that he cannot see himself in a poly relationship. I think it speaks very well of all of you that he was able to speak his mind, and that you and CLP were able to hear him. But I know that doesn't make it hurt any less.
If he loves you and desires a relationship with you, perhaps he sincerely considered poly and has now decided it won't work for him. Or perhaps he hoped you would dissolve your relationship with your Common Law Partner and become his partner, monogamously. Or perhaps he was willing to explore the possibilities, and this is where the exploration lead him.
If he went into the situation with his mind made up, and was only going through the motions of trying to make it work in order to manipulate the situation somehow, that's pretty low and you have every right to feel used and mislead. But perhaps he really didn't know at the outset how he would feel, and is now simply being open and honest about feeling the relationship has no future. And heartbreaking as it is, because he's decided your V is doomed, it is. I'm so sorry.