I'll add just a couple of thoughts of my own...
- It's not ok for your husband to continually disrespect your boundaries. He needs to provide a clear explanation for *why* this happened. Did he misunderstand, was he just horny, did he think it was unfair? Why did he text her when that was clearly counter to your request to him? If he's trying to go behind your back that is NOT ok and needs serious attention. On the other hand, if he just wanted to say "hey, sorry things got weird the other day", that just seems human.
- Some rules make sense in theory but don't work so well in practice. It doesn't excuse your husband's repeated overstepping, but it might be that these rules just don't make sense in practice. It may be helpful to think about *why* you want these rules. What are you afraid will happen if the two of them text, or if they continue fooling around in bed while you're there even though you've decided you're done? Are there other ways to either break down or work around these fears?
- It sounds like the third is in a really tough position and has done her best to respect you, I hope that you appreciate that and endeavor to work things out such that you don't just drop her because that's easier than sorting things out with your husband. Being unceremoniously dropped when the couple discovers that things are more complicated than they thought is one of the main reasons why bi poly women shy away from anything that looks like a "unicorn" role. That stuff hurts.