This problem is not uncommon in young children who struggle with the concept of using a potty/toilet when they have been used to using a nappy for nearly 3 years! What we do in this situation is to just leave the nappy open on the potty so when your little girl wants to wee you can sit her on the nappy on the potty and hopefully this will give her the confidence to let go and do a wee! Once she is happy to do this parents replace the nappy with toilet paper if necessary otherwise they just ‘forget’ to put the nappy on and sit the child straight onto the potty.
If she is not ready to sit on the potty with a nappy on just yet then leave her in knickers and when she looks like she wants to wee take her to the bathroom and put a nappy on (the cheapest one you can find) let her do a wee then take off the nappy and leave her again just in knickers – hopefully she will start to relax and go to the bathroom happily each time she needs a wee – once she is in a good routine re this you can then give her a choice – ‘do you want to sit on the potty/toilet with your nappy on or without your nappy?’ She needs to understand that sitting on the potty/toilet she will have to do eventually – but take it at her pace.
The good news is that she is very aware of needing to wee and she has very good control – so she is virtually there!