Well, many people disagree with my family's way of potty training, but we all had kids in underwear by 3, so it's up to you.
You tell them it's time to be done with diapers/pullups and you take them away. This is a challenge for mom, because it means cleaning up a lot of pee, but it shows kiddo that mom's in charge, not them. You take them regularly and reward them if they go. (Hugs, happy talk, dancing, high fives, and 4 M&Ms was my usual method, but it just depends on what the child responds to most.)
However, if the child peed on the floor, or in the pants, etc, there was stern talk, a firm "no", an uncaring change of clothing and no more attention. As they got older and proved that they could easily go by themselves and they chose not to, then they received a firm swat to the bare butt.
This is about them choosing not to do something they are capable of. You need to establish that now that she is a big girl she goes on the toilet. Along with that, you can give her other examples of what big girl things she does with mommy that her little siblings cannot do because they are too little.
Blessings on your journey!