Yeah, start by reading all the other threads about threesomes. There are (conservatively) dozens of them.
As I tend to have to sort out the aftermath of some 3 soms that have gone wrong I recommend the following 2 conditions. Make sure that your core relationship is 100% strong and that neither of you have jealousy traits .Anything less will be risky and not worth it .. Try and use the safe and controlled environment of a Swingers Club . This way you will meet like minded people without the danger of anyone developlng feelings for either of you .
Another condition I wanted to add . Make sure its what you both want and not just you .I trust that you have discussed this thoroughly with your partner as well .
New members, whose first post is about looking for sex in some way, always sounds like a fake post to me. . I know very few women who would go on a new forum with this thread as their first post...however, I know quite a few men who would.