Why is she SO flirty with them?

Dude I understand what your trying to say but look if she can have guys friend than you surely can have girl friends. Their is nothing wrong its just probably she has been knowing them for a longer time. But don't worry as they say " just cause I flirt doesn't mean am interested". But what the hey she still loves you that's what counts.

She probably views how she acts with them as normal behavior whereas anyone outside of the three of them may see it as shameless flirting. Because they are her best friends there is a bit more of a closeness than your average friend, but I would say not to worry because most likely she really does view them as just brothers. I'm very similar to your girlfriend in the respect of when I get really comfortable and close with people (and we are mutually aware it is not a date scenario) I can be viewed as other people as being highly flirtatious. And anytime someone points that out it boggles my mind because I've been viewing the situation from an entirely different perspective and my mind sincerely has not even gone there. Hope this helps! Best of luck with your situation!

OH! And before I forget! To address the her ignoring you thing, she most likely has know her two guy friends for a longer period of time and therefore they are her safety net in the situation where all of you are involved. She may be even be subconsciously trying to make them accept you and vice versa by trying to lighten up the atmosphere and show her friends that she still loves them and that she is proud to be with you!