Three years of use on the tires. At 15000 per year, thats 45000 miles. If they were a 50000 mile tire, she did pretty good. If they were a 90000 mile tire, not so good.
I'm not really sure what you are trying to tell us. All tires start with a certian tread depth, and can be used until a minimum is left (3/32, or until the top of Lincolns head shows if you use the penny test). Without knowing the mileage put on the tires, and the expected life of them, its hard to draw conclucions. As they gave you a 35% credit, I assume they wore out early, but that is not always the tires fault. Alignment, driveing style and many other factors can effect the life of a tire.
I once burned through a tire in 20000 miles by not noticing that I, or my wife had clipped a curb, and jacked up the alignment. Car still drove and braked in a straight line, but it ate up the inner tread. At 15000 mile rotation all appeared fine, at 20000, one was unserviceable.