The Mk3A and Mk3B are essentially the same helmet, the only difference being the Mk3A has no mask hooks, but the Mk3B does.
So really, they are two configurations of the same helmet, there isn't really a dedicated Mk3A or Mk3B, as it can be changed form one designation to the other by removing or adding mask hooks.
The Mk3A / Mk3B are fitted with the Mk2 Visor as per the Mk1A helmet.
This 1972 Helmets LTD brochure for the MK3A / Mk3B helmet confirms the presence of mask hooks makes the helmet a Mk3B:
The below exploded diagram from the above brochure shows a Mk3A helmet as there are no mask hooks present.
This image (estimated to be from 1972 to 1974) shows a Mk3B helmet worn by a Harrier GR1 pilot with a V1-Type oxygen mask, connected to a chest mounted oxygen regulator type 317A Mk2 (out of view).