I would stop buying nappies for day and encourage undie free at home. I would take her to the toilet at regular intervals and before leaving the house.

I would get a bucket of water next to the toilet and when she weeks explain the water is to wash it away. Encourage her to pour the water herself. When she is confident in that explain that's all the flush is...a magical button that pours the water for us.

I would avoid too many treats and just focus on one treat if she does all the wees in that day on the toilet ie if she doesn't have an accident.

Good luck! 3.5 is a tricky age for everything!