Have they ever done poly dating before? If it ok with them if you have other partners?
It is unrealistic to expect relationships with different people to develop in the same way at the same rate, so it's perfectly okay that you're falling for him more than her. It's okay to be involved with both of them but only have a dating-type relationship with one of them... you don't need to be "their" girlfriend (note my signature line). And if the idea was to be in a relationship and not just have sex, then of course it's okay for you to feel strongly for him.
However, it sounds like she may be uncomfortable with how fast things are moving. What is called for here is a big, honest, serious, sit-down talk, maybe with just you and her first and then with all three if you. Get it all out in the open. She doesn't need to be ashamed if this is shaping up much different than what she expected and it's causing her problems... but they can't get addressed unless they're brought to light.
I get that it's scary to bring this stuff up for fear he'll end up leaving you. But if it's just allowed to simmer under the surface it'll boil over into an explosion sooner or later and it'll be worse than if it was dealt with early. Talking now is your best chance for this to continue.
It might be really helpful for all three of you to read this, and there's lots of other great stuff on this site as well: http://www.morethantwo.com/coupledating.html