Ughhhh (shudder). I remember this from when my boys potty trained. Pee was easy to get the hang of, but I threw out more than a few pairs of undies!!!! Ick, for sure.
For whatever reason, my daughter didn't seem to have as much trouble.
I have to say, as a parent, I would want to keep up with the undies. BUT, as a provider, I would ask to go back to pull-ups until some more success is seen. Since she is 3, you can discuss with her why it's necessary. And talk about how, when she starts going poop into the potty, she can wear her big-girl undies again. Offer a lot of praise for learning to go pee in the potty. And just let her know that pooping in her undies is really yucky for everyone, so until she is pooping properly, she needs to wear the pull-ups. Maybe they can use the "cold alert" ones, so that they aren't comfortable to wear. That will hopefully stop any regression with the peeing.
That is, of course, assuming that she doesn't really WANT to wear the pull ups. Otherwise, I don't know if she will have incentive.