Potty training a 3 year old.

make her drink a lot and take her every now and then, with the twins (almost 3) we used butterflies stickers as a reward, one for a wee or a poo and three (WAO) if they could tell us they needed to go to the toilet. we also went for undies straight away so they understood the difference and that do a wee in undies is wrong basically...
make a big deal of flushing and putting wee wee in the toilet like a big girl
be ready for big accidents...yack

We're just getting into this with our oldest. Our plan is to "one day train" with nakedness and visits to the potty every hour she's awake. Then training pants all day until she can sleep/nap without wetting a few times. Then panties all day, training pants at night. Then panties all the time.

The hardest advice I've heard is "positive reinforcement" all the time you are potty training. Huge cheers for when she uses the potty correctly, but also not letting disappointment or discouragement show when they don't get it right.