I would like to have more than one partner at a time, but how do I ask or
express that to my girlfriend?
A: Oh that's easy. The next time when having sex with your girlfriend just say to her: "Wait for a second!". Then go and open the door, let the girl (or boy, I don't know) -you to want to have sex with at the same time- in and start to have sex with her (him) in front of your girlfriend. After a minute or so invite your girlfriend to join the threesome.
If your girlfriend is more sensitive, I suggest another procedure. Try to find out what she thinks of threesomes by asking her questions like "Do you think love and sex go together? Or are those two things completely independent?", "Ever thought of having sex with another girl?" and so on ... but never touch the real subject as long as you don't know that she maybe would like it.
After some of these questions you should know whether she would do it or not. If you still don't know come back and ask me again (but don't forget to tell me all your questions and her answers).