Advo,
Perhaps part of the reason that the GR's tend to use the 3 tank fit, is the availability of the 1500L tanks to them. Having worked both GR and F3 aircraft, whilst on F3's I only ever saw one set of 1500L tanks on station, which were painted in Camo, and similarly, when on GR's, the 2250L tanks were equally as rare and painted Barley Grey.
I have actually seen GR1's with a 3 x 2250L tank fit, however, this was whilst on the SAOEU, so was most likely a one off. Interestingly, the early RAPTOR pod prototype was a modified 2250L tank, painted white and fitted to the LH shoulder pylon.
As you say, the majority of the F3 fleet now have the shoulder pylon fuel points disabled, however, it would not require much to get them back in a serviceable condition. It would likely require a fleets worth of serviceable 28v actuators for the fuel cocks however, as these are the first places a sootie looks for spares in the event of an actuator failure elsewhere in the system!!
Flipflopman