There's no authoritative source, but I researched this a while ago, and various sources suggest that the Mk3 came out in 1985, and that the serial numbers in use then were in the 58,000 range (or thereabouts, give or take a thousand). I guesstimate that they only sold about 2-3,000 Mk3s.
There was a guy on here a while ago selling or valuing a "Mk3" with a much lower serial number, I think in the 40,000 range, which is definitely and early 80s Mk2. That, I think, was a bitsa, either a Mk2 with a Mk3 muzzle weight, or a Mk2 with a similar but contemporary one.