Beware of getting oneitis with this girl, I repeat, do not get oneitis. If she is fliritng with everyone while seeing you I would see this as a red flag. But she is within her right to do this, you've only been seeing her three weeks, you can't really see this as an exclusive thing.
Chances are there is not much you can do to change her personality if that's the way she is, so I would just do your best to pretend not to care. Don't let her disrespect you though. I might make mention of it to her if she is doing it right in front of you, but then you run the risk of appearing insecure, which is the worst possible case scenario. Probably the best thing you could do is not consider her a long term prospect, and look at options for spinning more plates.
There's also a question of what is she doing that you consider flirting? Is she giving kino to other guys (really bad sign)? Making suggestive comments? If she is just talking to them I don't think I would consider it flirting. There's nothing wrong with a little flirting even if you two were exclusive, but if she's a heavy flirter it's a huge red flag.
Just remember that if she is the type of girl who is always flirting with guys, it says more about her than about you. Don't let it undermine your confidence, because this would be a part of her problem, not a problem with you. Be the prize.