>>6514237I think the problem you are having here is that most of us have never had multiple partners at once, and many of us wouldn't. Like, it is a fine fantasy but I can only think of one person I would be comfortable enough with to be willing to bring in and I know they wouldn't want to so a threesome is completely off the table for now. My actual advice would be that you seem like you are not comfortable with it, don't do it.
But that doesn't seem like an option so we are defaulting back to our general advice. Have fun, do what feels good. Try to be confident and don't worry if you make mistakes, laugh it off. Sex is silly.
What you really need here is more information so you can make a plan. So much of successful bdsm is planning. You know your girlfriend well enough, you probably know how to push her buttons, you need to figure out what the two other girls are like.
Are they new to bdsm, do they have some experience?
Are they subs or switches?
What sort of things are they looking to do and have done to them?
Are they together or just friends?
Are either (or both) of them bi or gay?
Figure out what is on the table. Spanking? Is it going to be ok if you have one of the girls give the other oral? That sort of thing. These girls agreed to a foursome, you can probably go pretty far.
Have your girlfriend find these things out so you can use them. Have her be your assistant before and during the scene. Make plans with her, she arranged this, she obviously had some idea of how she wanted this to go, lean on that.
Handling one partner is hard enough as a top, juggling 3 is going to he really difficult. Don't be afraid to break the Dom illusion, especially with your girlfriend. You need her help, ask for it.
Now, for the actual plan. This is going to get complicated, so write it down so you can keep track of it. Keep it basic, you are a beginner at this, so don't get too fancy.