In addition to Arlo's comments on ordinance, the 1A is also faster at altitude (by as much as 10-15mph depending on altitude), lighter, and carries more fuel.
The F6F-3 has different canopy glass in front and back, as well as not having rocket rails. Also didn't have WEP, IIRC. Akin to the P-38G vs P-38J.
In AH, the 1A and 1D performance is very close.
No reason not to have the F6F-3, considering it served in numbers and so many other planes in game with multiple variants. If not, at least give us the option for the rear windows, like the B-25C glass choices.
