Hi, Wifee and welcome to the forum.
This is not something I've ever had experience with myself, so my advice is not going to be all that great, but have you thought about looking around in the "Sexuality" area of the bookstore? Usually those sections have books that can cover a range of issues in sexuality, not just "how-to" manuals. Since something like a threesome requires a great deal of trust and tact for all parties, I have to think that you could easily find some books that offer better advice on the "social" aspects rather than just the "sexual" aspects, if that makes sense.
In particular, I'd recommend Susan Quilliam's "The Joy of Sex - The Adventurous Lover." I think that's one that can easily be found in your local bookstore.