George .. the ''48'' is TPI .... or turns of thread (helix) per one inch of length. This is coarse thread version TPI - fine would be 56 for this dia.
The ''3'' is a size/dia designation. In the Unified National system, numbers are used up until 12, and then we go on fractional, with 1/4" first, and on up from there.
TPI increases as dia's get larger.
Help a bit?