Thanks for all your suggestions.

We talked this over and agreed that it could be fun and that we trust each other alot. However, we both agreed that no matter how hot the fantasy we couldn't predict how we would feel afterwards in the post orgasmic crash to reality. So we took this into account and decided together that adding someone else to our bedroom wasn't a good idea.
The fantasy might live on in our minds only but I'd rather have that mental one than mess up a marriage of years for a couple of blissful minutes of reality. 3 somes aren't for everyone and I'm happy for those who get to experience them with their loved ones.