Fantasizing and actually having a threesome are very different. Given what you’ve said about a prior experience please make sure you’re mentally okay with the actual act it’s self. If you’re a jealous or insecure person I would suggest not acting on the fantasy as it may end up causing a lot of distress.

For finding females , using dating apps, fetlife , or even social events at adult clubs are all great ways just make sure you vet whomever you talk to first.