On the M4A3 (75) the driver’s hatch and the commander’s hatch operate. on the M4A3 (76) the driver’s hatch, the commander’s hatch and the loader’s hatch all operate.
You can see in this photo of the M4A3 (75) that the driver’s hatch is open and the half driver from the add-on set fits on the shelf built in to accommodate him.
For those of you that might build our 1/30th scale Campaigns Miniatures M4A3 (75) kit, both of the hull hatches can be made to operate. There will be additional unpainted crew figure kits in different poses available later this year to compliment our armor.