I'm going to make a joke about people having a threesome.
Why would you make a joke about that?
Because it's a funny subject to talk about.
Telling vulgar sexual jokes can be amusing but not everyone appreciates hearing them. You should read the room and be aware of who your audience is in order not to offend anyone with your joke.
Don't worry. I am googling how threesome works.
Even if you are technically correct about what a threesome consist of, joking about it can still offend somebody. Maybe only tell it to people you know like those kinds of jokes.