This 3" MPT ABS pipe is available in an overall length of 3", 4" or 6". 

The unthreaded portion of the nipple is 1/2" in length on the 3" version, 1-1/2" on the 4" version, and 3-1/2" on the 6" version. 

Among its many uses, this product can be used to create a standoff between a holding tank and a valve mounting flange.