My girlfriend and I wish to have a 3some, should we?

You need to ensure both of you and the third person have equally shared trust as any amount of jealousy will ruin the moment. I've never had a threesome and the only opportunity to have one felt wrong since my first ex offered one to try and 'fix' that relationship (then she cheated on me anyway so it was done).

My second ex was hard enough to satisfy just in a twosome config.