Can you say "power struggle?" Your three year old can :). And if she's anything like most 3 year old girls she's really good at it.
1)Stop the struggle. Leave the whole issue alone. Put away the pretty underwear and stop talking about using the potty-- completely. And don't say anyhting to her, just stop. No negative comments, no hinting around, no talking about being a big girl... NOTHING.My intuition and experience training 3 kids, one of them the most stubborn girl you will ever meet :), tells me she will ask on her own. And when she does make the underwear, etc available but don't make a big production out of it. Be encouraging and helpful but not over the top involved.
2)It sounds to me like she can do the potty thing. And if #1 doesn't work here's somethign that worked with my sweet little stubborn princess. She gets two pair of underwear for the day; when they're "gone" there's no going anywhere out of the house.There's no playdates, errands with mom, trips to the zoo, etc. Every day is a new beginning and there is no negative discussion when underwear "gets used up" for the day. It's just "oops no more underwear today now we need to stay home." And make certain there is something planned for the afternoon. When she misses an activity because of "lack of underwear" don't make a big deal of it (avoid the struggle)--- "I guess we'll have to try that activity tomorrow." end of discussion. It took my sweetie one day of missing a playdate to understand the need for using the potty. THE KEY HERE IS THAT YOU ARE ABSOLUTELY CERTAIN SHE IS READY AND THAT IT IS TOTALLY A POWER THING THAT IS KEEPING HER FROM USING THE POTTY. If you have any doubts don't do this.