Optimal spacing is usually 2.5 to 3.5D, where D= pile diameter.  Some codes also specify a minimum spacing of 24 to 30 inches (regardless of pile diameter) so this may be where your "3 feet" came from.  Ideally, the piles should be spaced such that the load-carrying capacity of the group is not less than the sum of the bearing capacity of the individual piles.

Thanks! KAM

this is too late in the day maybe, but pile spacing is supposed to take cognisasance of pressure zone influences especially for friction type piles.

while min. spacing is recommended, often 3.14D ie pie *D an optimal spacing is necessary to achieve an economic pile cap.

Spacing requirement is not very critical for end bearing piles.
Also, for friction piles, there is a pile group efficiency component for the pile group action, depending on geometry of installation etc.(Feld's rule)


kay