Since threesomes have little to do with making a strong emotional connection with the third person, the Tinder format is probably more fitting.
After all it's all about the looks if it's just about sex, right?
It might not seem appropriate to ask for medical certificates but if it can be discussed it should. Deciding on the right person to join you is important for the enjoyment of the experience.
Decide as a couple which sort of person you want, so that you both know what you are getting into.
If they are fragile they might become emotionally attached to either you or your partner.