Thinking About Having A Threesome with My Girlfriend

I wish I could talk my girlfriend into having our first threesome with me. I have already picked the woman for us. Whether should I try to persuade my girlfriend into it? Some people say that threesome should not be forced to it, or it will ruin your relationship. I think they have some point in this. What if my girlfriend see me with another woman and cannot handle it. While some people say that most people are actually very open. They just need some push. What do you think?