Nonexistent. There used to be several grades of Specialist ranks.
They would be paid the same as the NCO rank in the same pay grade, but would not be NCOs. They would be SP4 (E4), SP5 (E5), SP6 (E6), SP7 (E7), SP8 (E8), and SP9 (E9). Eventually, most of these were done away with, leaving only the SP4 rank, which was known simply as Specialist, and the abbreviation changed to SPC.

There was never a Specialist 3 (SP3) - the only rank in that pay grade is Private First Class.