How to convice my girl to join me and another young lady in a threesome

If it taks convincing, I would not recommend having a threesome. She may say it's ok, and do it for you, but she is more likely than no going to resent you for having her do it. I could be the end of your relationship.

If she has a curiosity about it, then I suggest you and she talk extensivly about what you want to get out of the threesome, and if you both want a female partner. (She may prefer a male - if she does, so you still want to go through with it?) If that goes well, then you need to find the third party. This person, whomever it is, needs to be on the same page as you, be it that this is just sex and there is to be no contact other than for sex. Or, it's a friend, but she doesn't want you talking to her, or being alone with her after the fact. Or who cares, anyone can call, hang out, go shopping, recieve a blowjob from the third person whenever they want.

Once everyone is on the same page, have a great time!

You are playing with fire.

If you really care about this woman you could do more harm than good trying to do this.

Unless she is open to this trying to convince her is not the way to go.

If by talking about she says that she has had fantasies about it then proceed. But don't be surprised if you convince her that she might want to be with you and another man too......didn't think about that one did you?

best way is to play some porn movies showing this and get her response, give it time and see were it goes. Don't pressure her.

During sex use techniques to stimulate more than one or two areas on her body. To simulate having a third person during sex by using these techniques you might be able to convince her since she did respond to this. then the threesome porn movies might come into context better.