Personally, I would have never allowed less then 4 days. It can be very difficult to transition a child, on a 2 or 3 day a week schedule.
I always work it that if they were to do full time of 4 or 5 days then my rate is 45.00 a day. If your child only comes 3 days a week, then they pay 55.00 a day. So they essentially get an extra day for 15.00 dollars.