As part of the open communication with my husband this was one of the examples I gave about the sort of things my partner gets me to do.
Look, how would husband know if you did not tell? Does partner tell him? Or you tell him? Then you are good. It is learned openly in conversation, right? There is not some kind of TMI rule in place? Then there is no actual betrayal happening here.
Unless YOU are betraying YOURSELF by agreeing to an "open communication" policy that does not actually sit well with you. Maybe you prefer a policy that is more like "TMI is TMI. You can know when we have sex, and sex health info, but not the sex details of what we do because that makes me feel weird."
Obey your own limits! Do not compromise yourself. Just because the two guys are fine without a TMI wall, doesn't mean YOU are. And if YOU are not, then the TRIO AS A WHOLE is not even though certain people in it are. The TRIO is not. And it cannot be forced.
But if you like your totally open communication policy? Have no TMI limit you are denying/ignoring inside you?
No problems are here other than you needing time to get used to this new dynamic then. Enjoy it freely -- there is no betraying if you are all happy with open communication policy.