Where I live none of the pre schools will take a child unless they're 100% completely toilet trained and confident in it. Otherwise, forget it. Any one in nappies definitely need not apply.
This is illegal.
I am always surprised on threads like these, that it is seen as acceptable to not support a child in one of the prime areas of development at such a young age.
Some children at 3 will be developmentally ready and able in areas of self care (toileting, hand washing, getting dressed etc) others need a little more support - hence why it is in the EYFS. We don't take other areas of development and say, children that haven't achieved that particular area can't come to preschool....its bizarre.
For some children it is just that it takes them a little longer, and they may need to ask an adult for help (another important skill), for others it may be that there are medical issues (a LOT of children have constipation at this age) and for some it may become apparent there are additional needs
It should be perfectly possible to support a child and help them learn how to wipe their bottom etc. I would not send my child to a setting that had a problem with this.
By the way - games where you pass a bean bag through the legs and round your back are good for helping with the dexterity you need to wipe your own bottom. Also setting out the steps you need to take when you finish very simply - it is a tricky process to learn!