Found Nyc's links!
[With this one, you can select "Relationship Issues" after you narrow it down for your zip code.]
When I hear it's "only" happened x number of times, it's like hearing "he's only cheated on me 20 times in the last 30 years" or "he's only stolen from me 20 times in the last 30 years" etc. That may sound extreme, and I promise I'm not trying to vilify your husband, I'm sure he truly does love and cherish and respect you, it's just that to me this is a very serious issue that corrodes trust and a sense of safety. Once or twice is bad enough.
That said, I don't think we'll get far arguing the semantics of what's abusive versus what's "just" unhealthy, so I'm not going to press on that point -- I'm just really glad that you realize that there's a serious problem here and that it needs to be addressed. That alone can be hard enough, so good on you for facing it.
Everyone who gives advice on poly will say you need a strong, durable foundation to even consider it, because it will test you (as you're already seeing). It's graduate-level relationship stuff, is one way to put it. To be frank, you guys aren't there yet. If you think it would hurt to lose T&S now, imagine how much worse it would be if you'd gotten to the point where you had all decided to let love blossom, they had become a vital party of your heart... and THEN R freaked and unilaterally pulled the plug. It's one thing to lose a relationship because it just doesn't work out, you can get some closure there, but there's no closure to be had when a healthy relationship is veto'ed.
That is not to say you should dump T&S to protect yourself or "for their own good", far from it! I don't believe in dismissing a good thing out of fear when there's a chance to work things out. But I do think things should maybe cool off a little for now, maybe focus more on the friendship side. Be honest with them that you're having some relationship issues that you're working on, and that it's not their faults at all and you both care for them... hopefully if they're good friends they'll respect you for that and be patient!