Down that road you will find pain and misery and the high potential for a break-up. Men can have sex with anything that moves and not become emotionally attached... most men. Women on the other hand have "feelings" sex is not just a physical act it is an emotional act and women tend to get confused about love and what sex means... most women. The risk here is obviously you are young, are in a temporary (at best) relationship where you have not developed strong enough emotional attachments to your GF to take the risk. The Friends of your GF may not be swinger material - they may fall in love with your or demand commitment or something other than just sex. That will only open the trap door which is a long cold fall into mistrusts and regrets. Aside from all of that, if you two are open and honest about trying a trio situation then both of you need to approach the intended target of such affections. Together you need to ask and discuss and then lay out the specifics of what this is, what is included and how people will treat each other after the event. Multiple partner relationships like this only work if all the partners are willing and most importantly able to separate "feelings" from the physical sex act. If any one of them are unable to keep emotions out of this, attachments as out lined before the act, it leads to all sorts of issues and problems which frankly steals any pleasure you may get from the sex.

If you push the issue your girlfriend may decide that she no longer wants a threesome. Obviously, that is not the outcome that you are seeking. I hope neither of these girls have a boyfriend. My answer to your other question explains my thoughts on that. A good way to break the ice on this is to let her catch you checking another girl out. Instead of getting defensive you could ask her if she thought that the girl would work for a threesome. If she says yes, work on that girl, if she says no- thats fine, but try and get some feedback, so you know more of what she is looking for. Find a girl she wants to be with, and she be presurring you for it! Best of Luck!!