They said that the 3 day limit is somewhat at their discretion - if you were planning on going for 4 days and phoned and asked they said it would probably be OK.
I did speak to them the year before and they didn't have any minimum hours policy at all then, but presumably either abuse or lack of aircraft availibility for others has forced them to bring it in.
It is slightly offputting - I've been looking at rental in the States recently and it seems the norm out there.