MGS using MOC can get, using a technique called CPROTO - about 1.5m resolution cross-track and about 0.7m resolution down track - with just a b'n'w CCD - and has a 2048 pixel CCD giving about 3km image width. Normally it's approx 1.5 x 1.5m

MRO using HiRSIE can get 0.3m cross track AND down track - with images 20,000 pixels across, thats 6km across. The whole width is available in Red, and the central 4000 pixels ( about 1.25 km ) are also imaged in nIR and Blue Green at the same time

So - for any given pixel for normal MGS MOC imaging - you get about 25 pixels from HiRISE - in three channels for the centre of the image - and you get about 12 pixels from HiRISE for any one pixel of MOC using CPROTO.

Who ever has said the values 3 inches is talking out of their secondary mirror.

Doug