I am not sure which helmet that you are showing from a front view of the front plate?
G R 3
could be Grenadier regt 3
but it takes a special Grenadier plate
Steve
If the plate is original to the helmet, a couple of things might have happened:
1) this really was to a grenadier regiment as suggested, but due to the lack of special plates or just to simplify supplies, it was given a standard plate
2) it could also be that this did have the original plate and was assigned to a grenadier regiment, but then got refurbished at some point and turned into a regular m15 pickelhaube and the depot just left the markings as is