Did they?
That diagram appears exactly as captioned in the Crowood book by Kev Darling on the Seafire p61. Yes it purports to have been provided by the FAAM (Fleet Air Arm Museum) but does it represent a Seafire III wing? In Appendix IV in that book then goes on to list the internal fuel capacities as follows:-
Merlin powered - 48gal (fuselage tp tank) 37gal (fuselage lower) for a total of 85gals.
Griffon powered (Mk.XV & XVII) - 80.5gal (fuselage tanks) 19.5gal (wing root tanks)
This from David Brown's "The Seafire. The Spitfire that went to Sea" about the development of the Griffon powered Seafire XV:-
"....To the Spitfire VB-based fuselage and the LIII folding wings were added ...... wing root fuel tanks from the Spitfire IX ....."
And from "Spitfire the History" in the section on the development of the Seafire XV and the changes to get to that mark:-
"F Mk IX Mareng fuel cells were specified for internal wing fuel."
And that same diagram, without the blurb in the bottom left hand corner appears, but with some additional details on spar construction, not in the section on the Mk.III, but in the section on the Mk.XV. See page 554.
David Brown lists the internal fuel capacity as 85gals for ALL Merlin Seafires. For the Seafire XV & XVII 80.5gals in fuselage tanks and 19.5gals in wing-root tanks. These figures are supported by "Spitfire the History", and, as pointed out by Geoffrey Sinclair, the Pilots Notes.
So other than the diagram that you posted, I can't put my hands on any evidence that the Seafire III was fitted with wing tanks. Everything else points to the contrary.
As for the fitting of Vokes air filters to the Seafires, David Brown notes in the section on development of the Seafire IIc, that a number of aircraft were fitted with them but then a decision was taken to delete it from later production aircraft "and the risk of excessive engine wear was accepted". So you will find photos of Seafire IIc on Formidable in 1943 fitted with the bulky Vokes filters. But L.IIc/LRIIc (both production line and by conversion) and early production Mk.III did without any filter at all and the photo evidence supports that. The smaller filter, as fitted to Spitfire VIII/IX etc, was then fitted on the production line from later in 1944 and probably retrofitted to earlier surviving aircraft. So I would question whether any Cunliffe Owen Mk.III were actually fitted with the Vokes filter before the neater version was introduced around Sept 1944.
The Spitfire 'C' wing, as used on the Seafire IIc/LIIc/LRIIc/early fixed wing Mk.III was modified internally for later Mk.III, including deletion of the outboard cannon bay and blast tube. Not sure when that occurred but it must have been later in 1943 or even earl 1944 as there are photos around of early folding wing F.III with cannon stubs. The first 30 or so Mk.III got fixed wings. It was Oct/Nov 1943 before folding wing versions began coming off the production lines.
As for the ejector exhausts this was a later introduction. Even early F.III produced in late 1943 didn't have them. At the moment I suspect that they were introduced initially on the low level rated Merlin engined aircraft like the L.IIc/LR.IIc and which then formed the bulk of later Mk.III production (i.e. all bar about the first 103 Mk.III). David Brown puts the first conversions of Mk.IIc to L.IIc as March 1943 but as the Barracuda had priority for the low level rated Merlin 32 it was mid-May 1943 before the first squadron received its full complement, but by Aug 76 were in service.
As for the change to the slim cannon bulges that was being made on Spitfires from early 1943 (Mk.VIII for example from sometime around Jan-Mar). There are photos of early Seafire Mk.IIc with the broad bulges.