Nicer, pilot friendly, birdcage canopy without the head-board, and the option of 4xMGs (cannonless) which gave it a weight save over the E4.
There were E3s with 4xMGs and the 20mm through the prop hub, but they couldn't get the MGFF to reliably fire when housed in the hub at the time.
^ this
A few early E-3's still had the guns of the E-1: 2 x 7mm in the nose plus 2 x 7mm in the wings. Messerschmitt was unable to get the 20mm
motorkannon to work until the later F-1 version, near the end of the year in 1940.
As for adding more variants for use in events:
The E-1 could be matched against either the Dewoitine d.520 or Morane-Saulnier M.S.406 in a "Battle of France" event.
The E-7 would find use in both Russian Front and North Africa events as the principal
jagdbomber for the Axis.