"do you make them go by the schedule they give you or do you just always keep those 3 days available for them?"
I would be available for the days they contracted and paid for. I also would not allow them to switch days during the week.
When I offered part-time slots I only had two options; M+W+F or T+TH. It was splitting up one slot. Anything else takes up a full-time slot at part-time rates.
Now they pay one rate and are welcome to attend as often or as little as they like. No schedules required.