To us, this is a fantasy and will always be that. We've known three couples over the course of time who were involved in threesomes (two of those marriages ended over it and the third struggled after). A lot of people seem to manage it just fine, but I think if you really have to convince your partner or talk them into it, that may not be a good sign.
I think the fact that you have firm no from her means she probably doesn’t want you to bring it up again. I would love to have a MFM threesome just so I could watch my wife suck & fuck another man. When I brought it up she was a definite NO. I haven’t spoke of it again.
I think that's nothing but trouble brewing. Find something fun to do in your marriage besides trying to fuck it up.