YANBU, OP. I would be furious!
Three year olds being stubborn, wilfully independant and denying the obvious... par for the course, really, I should think.
Of course the child is saying she can manage alone or denying there's a problem. She wants to be a big girl and 'big girls' don't need help.
Except that she does and a nursery, a good nursery, would be aware of that. I am astounded at any nursery that would ignore the issue like this, to the point of compromising a child's health.
As for not for OFSTED: If not them, then who?
It may or may not be 'only one child' but since ' every child matters', surely they have a duty to investigate whether one child or fifty? Surely, the point of a body like OFTSED is to catch issues like this when it is only one child, and before it damages lots of children?
Or have I misunderstood, and they really are completely pointless and lazy? Yes, they're there to set standards in Education but in the Early Years, hygiene is a standard that children need to be taught. That isn't happening in this case, so they've failed to meet the standard for education.
I would definitely report.
I would also, btw, be pulling my child ASAP. Can you take time off work, OP, or so you have family who could help?