Threesome

My S/O and I have talked about wanting to have a 3 sum many times. I mentioned that we could find a girl on tinder or other sites but they said that they would not want to plan it, and that it would need to happen naturally. In my opinion, I don’t think that would work, unless we did something with one of our friends... but we don’t want that. I believe that you need to plan these types of things. Especially because you need to make sure all parties are comfortable before you do it. What do you guys think? Should it be planned? Or should it happen randomly at a party or something (if your lucky)?