Do a web search for "Lifestyle" or "Swinger" sites. From there you can drill down (pun intended) to the sub-sexual proclivity of bisexual web sites that cater specifically to lifestyle or swinger folks into MMF, FMF, MFM, group, etc. (I hate the term "swinger" -- reminds me of dull, middle-age straight couples from Mill Valley in the late 60s early 70s who drove Volvos and tried to play catch up with the zeitgeist of the era or recapture their youth. Ewwww, fer ick!)
Anyway, they are out there. The more serious sites will embrace the term "lifestyle" rather than swinger. Good luck and play safe.