If your ok with having a threesome, just set some ground rules together. Tell him you don't want him having sex with her and that you want to fool around with the girl

Perhaps imply that you want to have sex with the woman while he watches and when he gets involved he ducks you while you go down on the girl. Something like that. Problem is with. Threesome it's going to me tough if you don't want to share your husband. You will have to make sure with him and the woman that the two of them don't touch.

If you want to avoid a threesome perhaps bring up the convo with him again. Let him know that you didn't mean threesome and that you yourself want to have sex with a woman. Either you want to do it on your own without him around is sometimes you will have to discuss with him. But if your ok with it your husband could watch even record your experience. It could help convince him as well.

Just don't go into something that your not 100% sure of. It will come back to bite you