Plate 3 was registered on 09 May 1840.  This plate was the first of the Penny Black Plates to be destroyed (October 1841) after being in service for only five months.  Evidently this is because the plate wore rapidly. The stamps from this plate often have a dull grayish appearance and could be easily mistaken for Plate 1a if not for the the Ray Flaw which appears on Plate 1a.  This Plate does not have a Ray Flaw, and has very few other means of identification: there are no known re-entries, few guide lines, double letters or dots.  This plate is sometimes refereed to as the "Tidy Plate."

The fact that Plate 3 has no obvious characteristics means one must use the process of elimination:  It does not have Ray Flaws which eliminates Plates 1a, 1b, 2, and 11.  It does not have the O Flaw which eliminates Plates 7, 8, 9, and 10.  This leaves only Plates 3, 4, 5, and 6.   We will  learn when looking at Plates 4 and 6 that many stamps from these plates tend to have dots.  Plate 5 very often has a characteristic bottom frame lines bulge on either side of the "O" of "ONE."  My own experience is that the stamps from Plates 4, 5,  and 6 are very often in a fairly dark shade of black.  It is therefore possible, through this process of elimination, to arrive at the probability of the stamp originating from Plate 3.  This could of course be confirmed through the measurement of the check letters using The Plating of the Penny 1840-1864, Volume 1.

Another useful characteristic which helps at least identify some of the stamps lies in the nature of the check letters.  The letters K, M, P and A are unique to this plate:  The K and M are narrow in comparison to the broader letters of the other plates.  The letter P has a small loop in comparison to the normally large loop.  The letter A is somewhat squat and is sometimes blind (the upper portion is filled with ink as in the below example.
 

Example  QJ	ExampleTF

Example of a Blind "A"  in south east corner