Hello, glad you like my posts. It’s definitely harder to find clean not sketchy people for threesomes. It’s much easier for find fellow couples who swing.
The best site is http://www.swinglifestyle.com because it is the biggest and has the most members. We did meet our first couple via FetLife but it’s not really setup as a hookup/dating site and It’s not swinger focused. You might also find clubs in your area via Swing Lifestyle where you can meet people.
Good luck!