Is it cheating? That really depends on how you see cheating. It depends on how the 3some happened, and what led up to it.

Edit: I did not pay attention to the fact that this woman is married, my bad. You helped her cheat on her husband, moving out or not, so that is pretty shitty. She cheated for sure, but did your husband cheat on you, read below.

Here is basically how I see the situation. You and him both slept with this woman. You had expectations that you didn't communicate, and he had different expectations that he didn't communicate. Did you or him say anything about it being fun/talk about this in the future, talk about enjoying her in your relationship? If any of this happened, I'd guess that he thought you enjoyed the 3 of you involved together as he did.

Here are the things I consider important, if it was me which are positive: 1) You consented and enjoy a sexual encounter with a friend with your husband together 2)You were happy that your husband was making her happy, and I've assumed he knew that is how you felt about it, which also sounds like that made you happy. 3) He told you immediately, and by no means intended to hurt you, and thought you'd be happy about the continued involvement in your relationship with her. 4)You know he is committed to you, respects you, trusts you and loves you

Here are the things that I think are bad: 1)He had sex with someone who you had slept with the night before and you are hurt: this is something that you must talk to him about 2)Neither of you talked about the boundaries, so if in general you both have been pretty trusting with each other, you took assumptions for granted, and so did he, and the misunderstanding of what happened last night has led to a mistake

IMO I don't consider what he did cheating, I think that he assumed that the involvement with her would continue, and you had no expectations that it would continue. I think this situation is something that you need to tell him how you felt, without assigning blame, and ask him how he viewed the situation with this woman. Then explain how you viewed the situation after the 3some. Then talk about how you wish to continue. The important things I see is that you have no reason to believe that he will do something like this again, without discussing it first, you can trust him, and he didn't do anything that he thought would harm your relationship. Good Luck either way, and discuss boundaries in the future.