3 holes? oh boy!, you are realy lost there. I wonder what will be the use of the third one. Well, my practical advice is that you go and find some porn pics in the web (for educacional purpose of course, that should not be to so hard, since there are trillions of them) and take a good look, there are plenty of vaginas there. That's just so you can get an idea of what you will find on your g/f.
I hope some one here can give you a better help. Good luck
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