My girlfriend has started to frequently talked of a threesome, MMF, with me being one of the M. How do I go about this?

Tough one. If you are into the MMF three way, I don't recommend using someone you know well or see often. In fact most guys you know may not be interested in the first place out of fear of "crossing swords". It will change your relationship with the friend and not usually in a positive manner. I can appreciate you wanting to make your partner happy, but this is one of those things you have to be just as into as her to actually do it. After all there are plenty of other ways to make her happy.

You could slow the approach and buy a dildo to use while you are having sex with her to substitute the second guy and then just watch more porn together.

If you're not comfortable with it then tell her that. It's a bit weird to me that she already knows who she wants the guy to be. Maybe she just wants to sleep with him and is using it as an excuse?

I'd tell her sorry, that is not happening. End of discussion. Your apprehensiveness to the idea is the reason why you shouldn't do it.

Its all filthy sin so do not get involved.Think if shes worth dating if this is what shes into.

This is wrong. If you love each other, you should not be sharing your bodies with others.