I don't have my PRR Electric Locomotive Operating Instructions book at hand but I'm nearly certain that it contained instructions that the controller should not be operated beyond a certain notch when 3 GG1s were MUed together.
I can't tell from the photograph whether the motors were pulling a train or running light. Running light would require very little current.
I would tend to believe this shot shows a power-balancing light move, as I have never seen evidence of more that three GG1s on a freight train, and no more than two on a passenger train. If so, probably only one unit in the photo was under power. The others had their pantographs raised but were probably not providing any traction. Whenever I saw light engine moves all rear pantographs were always raised. Pennsys (and other railroads') use of the rear pantograph was strategic. I they used the lead pantograph and it somehow got ripped from the roof it would likely take the rear pantograph with it. By using the rear pantograph, the motor could continue by raising the front one in case of a failure. For some reason the Milwaukee did not follow that convention and seemed to use the front pantograph.
Apart from the grades at the B&P tunnel in Baltimore, where a helper engine was stationed to assist freights, there were no grades that would require more than the 218,000 lbs of TE that a 3 motor consist (with 90 MPH gearing) could provide.
On a related note, I never saw a photograph of more than two New Haven EP-4s or EF-3s MUed together on a train. They were essentially the New Haven answers to the GG1s, in passenger and freight service respectively.
Also note that there is no hyphen in GG1 (or any PRR locomotive) but the New Haven designations used hyphens.