S.W.
Don't ask her, their answer will always be "no". Just make sitting on the potty a routine. Have her come in the bathroom with you each time. Set a schedule just like they do at daycares - morning, before/after meals & snacks, before bath, before bed. Have her use her potty to sit on while you run the water in the bathtub or the sink (my mom's trick and it works!), talk with her about what a great thing it is to be a "big girl", read books about it (yes, 3 year-olds can understand books written on this subject just for them), make sitting on the potty fun whether she goes or not (book or toy to hold). Give her lots of water and juice to drink so she can't hold it.
I never used rewards (other than a "good job!") but some kids may respond to them. I didn't wait for her to "be ready", I brought out the potty at 14 months and started the process really gradually until she was completely trained at 26 months (no accidents). Three year olds are past ready. Start again and this time, no going back.