Hey there. Lots of experience in this area, maybe I can help. The alternation between distance from you and then wanting close proximity to you (no to sex, then lots of close hugging) is indicative of his frame of mind.

It sounds like the experience of the threesome dug up emotions in him he was unprepared for, and dealing with them is giving him a see-saw mindset, which you are seeing manifest in his actions.

The best thing you can do is to communicate to him about how special he is to you, how the experience brought you closer to him, how much you treasure his confidence and trust in you to allow that level of intense play, etc. Reassure him that while it was fun, that he is your partner and that hasn't changed. It sounds from your post that your man is struggling and signaling for some reassurance - go above and beyond to provide it, and things should be better.

Also, perhaps a discussion of comfort and boundaries is in order. Talk to him. Maybe he was ok with watching you make out with another guy, but no sex. Or maybe just sex, no kissing, or maybe just hands and mouths.. Etc etc. The combinations for peoples comfort levels are unique, and maybe watching you get full on with another man has challenged his comfort zone. A talk about boundaries is most definitely in order of you ever plan to do it again for sure.

Bonus note: After playing with others, my partner and I would often have what we termed "reclamation time", a session following as closely as possible to the event where we focus on each other as intensely and passionately as we can, reassure each other of our love for one another, and talk about how trusting each other in this was strengthens our bond and brings us closer together. Give it a try, and good luck if you guys do it again!