You've already forced her and made it unpleasant. Lighten up a bit, it's more about you not wanting to change a 3yo, and less about when she's ready... not to be harsh, but seriously, it takes time and when they are mentally ready for it, they will do it. be patient w/ her, it's less frustrating for everyone involved that way. :)
Edited: Just wanted to edit this publically to say that we, as mommies, esp on this group, are a great group of ladies and i want to just apologize if i sound harsh above, don't mean to, but do think that developmentally, when a child is ready, a child is ready. They are usually physically ready before being mentally ready, and however we all deal with it, we all do know it's frustrating but eventually it does work if we are consistent and use praise and even the reward system if it works, because lawd knows, the cost of diapers is $$$$, and the thought of having to change my 3.5 yo's man-sized poops is getting harder by the day (not to mention smellier), and that if there is some magic formula anyone knows to get a child to just "get it", i'd love to know it :) My son has reverted back to hiding behind the sofa or somewhere and then lying when i ask, proving (like mentioned by someone above) he is in control of something in his life. I am guessing that like many boys, it will take until he's about 4, and pray that w/ my daughter, 23 months old now, that she'll be easier than my son... GOOD LUCK to us all, we are all great mommies :)