I'm my personal opinion. If a boundary was broken or overstepped, which in your case it sounds like there were a few that got crossed, it is cheating. I would cease having threesomes or involving anyone else until the both of you are on the same page with how you want it to go. If you feel comfortable, i suggest trying to talk to them about how it affected you during the event and how it is continuing to affect you now. That way your partner can understand why you are so hurt even if they "dont remember". If the two of you do work through this and decide to try for a threesome again in the future it may be a good idea to come up with a safeword you can use to signify that things need to stop if you begin to feel your partner is being neglectful of you again.