So sorry you are having old wounds poked at and having to endure new info. For 1, it is not too late to say anything, you just found out that not only did he conduct himself like that, he continued to hide and lie after going through the 1st A with you. Im sure his behavior was on the list of things he knew he could not do and he chose to do it to you, with your "friend" and then play house with you like he was a good boy in R.
If you feel certain that it was a singular incident, you can and should talk to him, you know it will just eat at you if you do not. However, if you question other actions now, maybe you should do a little digging, but only you know your H's actions and can make that call. Im not suggesting you dig and dig if you truly think things are good, because 3 years ago it is entirely possible things are going better now.
As for the friends, also your call. I would find it hard to consider someone a freind if they were willing to behave that way with my husband knowing they could devastate me and my family if I knew.
((beyondsad)) The people who should have your back and want you happy unfortunately are the ones who are carelessly throwing the knives in your back as you walk away.
I hope that if you do approach your husband he is remorseful and supportive of you being hurt and handles it without trying to downplay his actions.