I am not aware of any "max limits" in the regulations or in black law. However, I think the FAA could make an augment that 3000' is reckless and a risk to the National Airspace System. At that distance, one could argue that an R/C pilot cannot see the model and full scale aircraft well enough to meet the "see and avoid" standards under VFR rules. As I said before, if a mid-air collision with a full scale aircraft occurred at that altitude, they pilot of the model aircraft would be in a heap of trouble (far more serious than a $10,000 fine.)