How to tell a girl you are looking for a threesome?

Do you mean, how do you tell a girl you are looking for a threesome?

You come right out and ask her

If she isn't into it it don't matter how you word it

Just tell her "You're not enough. I want my fantasy world to be reality. I want a threesome. I know it will probably ruin our relationship, but obviously the fact that I want to add someone else into our sex life, tells you that our relationship doesn't mean that much to me. So, will you do it"?

Then she gets to decide.

I will assume this is some girl, other than a wife or girlfriend, someone you just want to hook up with for sex.

And what type of threesome, two guys, the girl may like that more. Two girls, what do you expect her to do,?

Yeah maybe you should start with you, her and another guy to get her with the idea first.

I bet she may like watching you with the other man, before she joins in,
Seems the guys have no issue thinking the girls should do this, but make it two guys doing the same thing, then they don't feel the same.

And lets be honest, you only have one penis, and can only do one girl at a time anyway, what is the other girl suppose to do, while you are having sex with the other one?

What is the other girl suppose to do, while you are having sex with the other one?

She is supposed to watch and wonder what did she ever see in him that he could go after another girl so easily

Don't. Call an escort company & cough up.

May I ask how old you and the other people involved are?

By the use of the term 'girl', I am concerned that you or she is underage and it is policy to not give advice on having sex to minors.

How old are you and how old is this "girl"?