You mentioned a diaper. For starters, you have to be consistent: either she is too big for diapers or she wears them. You have to show her that you believe her able to take responsibility for her body. So, for starters, I'd throw out the diapers. Second, reward her with something she really enjoys that is cheap when she takes herself to the potty.
When my daughter shows resistance (mind you, my daughter is 22 months), what I do depends on when it is. We have certain rules: we go before we eat, before we leave the house, etc. If it is at a "rule time," I try to distract her while picking her up and bringing her to the potty. This won't work with your 3 year old, but maybe you could do a distraction of sorts by throwing something she really likes to do in as an option after she goes? If my daughter is really resisting and it isn't rule time, I just remind her that pee goes in the potty, telling her I don't want her to wet her pants, but also telling her that I trust her to let me know.
Regressions: they always make regressions before giant steps forward, and it's our job to just clean-up the messes and hang in there.