Yes, what MonoVCPHG and BreathesGirl wrote. Poly is not just one thing -- it's not just triads, not just group living, not just anything. It takes many forms.
For instance, I don't hang out much with my GF's husband. I've met him and like him, we just move on different planes. And she did things sometimes socially with one of his past GFs, but not with the current one. C'est l'poly.
Big kudos to your husband for working with you and R instead of rejecting the relationship. I mean, BIG kudos! I applaud his generosity and strength of character. (Tell him I said that.)
It sounds like you are the socially skilled one in the vee. That puts you in a position not only of receiving a lot from two men, but of being able to provide a lot -- and with that ability comes possible responsibility. If you want them to relate more to each other then it may be that you are going to have to be both the glue the sticks them together and the oil that minimizes their friction.
My personal feeling tends to be that when the people on the legs of the vee know each other and can be friendly the whole structure is more stable.
We often fear what we don't know, and if your husband doesn't know R then he may fear his influence on you. Also, we tend to make guesses about people when we don't have full information, and that can lead to real misunderstandings -- your husband may guess that R is one way, and start acting on that assumption, when in fact R is not that way at all.
So it will probably work better if they can be at least casually friendly. They don't have to go out drinking every Friday, you can just have a 3-person dinner every now and then. Talk together. Talk is good. But you may have to be the one to arrange and mediate. Good luck!
On your husband finding a girlfriend: Tell him it's difficult but not impossible. My personal perception is that most women in the general population who want to date view poly men as cheating playahs and are NOT interested. Ironically, I think men looking to date view poly women as easy sluts, and are VERY interested. That's my personal guess about the general population's perceptions, at least in smaller communities like mine. We live in a mono culture which does not understand poly very well at all.
If he is really interested in comparing notes on finding and dating women he can PM me, either here or on the dating site OkCupid. It might help him not to pull the plug if he can realize that it's a process and never a finished, finalized thing. Your relationships will never be exactly the way they were yesterday.
You know, there's the famous metaphor about time being like a river: you can never step into the same water twice, because the water is always flowing past you and away. The Argentine author Jorge Luis Borges turned that around: he said we are like the river, we are always flowing and changing. And that's the way relationships are too.
So it will change. We can't always control or even foresee the changes. But thinking of it as a static thing which must be accepted or rejected because it has one fixed shape is probably a mistake. It does not have a fixed shape. It changes.