It's typical that there's a preference for pre-school students in the 3-6 y.o class to be independent with toileting. It isn't typical to push or shame.
A 3-6 y.o. class often has fewer instructors/caregivers than an 18month-3 y.o. class. They are less able to provide one-on-one care for feeding, toileting/diaper changes, etc.
It's possible that this 3 y.o. is more than ready for the work in the 3-6 y.o class and would fit in better with the older students, but he's not able to move yet. It would be a frustrating situation for everyone. I'm not sure you can do anything about it, other than withhold judgement and let the parents and teachers manage.