Yes, buying three one-day-tickets was cheaper than buying one three-day-ticket. This quite unusual difference was justified with an added value: “Allows for free accompaniment of 1 child up to the age of 15.”
The fares have changed after May 2011 and now, as expected, the three-day-ticket is cheaper than three one-day-tickets. I had to dig in the WayBack Machine to find the fares from last year. Today, your child of 6-15 years has to buy for each day an own ticket for 55 CZK.