Threesome

I think I want a theesome. Just to try it out.. I want it with another girl but I'm scared my fiancé will like it too much and I couldn't stand seeing him have sex with another girl. I'm the only one he's been with and i don't want him having a taste of another girl ya no? The girl would be there for me kinda. And maybe just oral for him? I'm not sure. I really want to try it but I'm just scared... Any suggestions?