What sort of threesome is this he's after? You saying you're not gay, he wants to see you with other women? Can you turn the tables tell him it has to go the other way too, you want to see him with another man? That may just shut the idea down.
How old are the two of you? You did this to please him, not because it's something that turned you on? How is it he has a dozen female chat friends that he feels would be willing to do this? It sounds like you are pretty tollerant and open to him having sexual conversations with other women.
Can you tell us why you don't want to lose him? What makes you think he is good marriage material?