Is she not caring about being wet/dirty in Pampers/Pullups or in cloth training pants/underwear? At 3 1/2, I would stop making this a choice. I don't believe that 3 1/2 year olds are "not ready" - they are simply opting not to do it. If you want it to be an expected behavior, then you need to make it a rule with a consequence. I wouldn't do this for a toddler, but she is 3 1/2. I'd put her in cloth and take her to the toilet every hour or hour and a half. I wouldn't ask if she has to or wants to go, it's not a choice. Unless she has a medical issue, at 3 1/2 there is no reason why she shouldn't be fully toilet trained. I wouldnt' even use a potty, those are for toddlers, I would go straight to the big toilet (my kids used the big toilet at 3 years old). If you want her to do this, then you need to step in, take control, be the authority and not give her any options.
I wouldn't do any bribing - that shows her this is something where she has a choice, and something that she can choose to do or not, that she has the control. It also makes it seem like something she is doing to please you rather than behavior that is just expected.
Good luck