While in the M4A3 (W) 76mm, you can engage most tanks out to 2000 yards safely. Beyond that you are almost helpless vs the Tiger, Panther, and T34. Remind yourself that you have a good main gun, but relatively low armor comparatively. The 76mm is good to the 2000 yard mark vs the heavily armored tanks , and as far as you can see vs the others. Your biggest asset in either M4A3 is your reload rate (3.6 seconds), the thorn in your side is your armor. Most anything can punch through even at the longer ranges.
#1. Try and get within 2000 yards of the Panther and T34, and even closer vs the Tiger. Even then their armor just may work.

Offensively, the M4 76mm is the best mid ranged offensive tank (vs other tanks) in the game, the combination of the penetration and reload make it first rate.
#2. Your armor is on the low end of the scale, so you should be comfortable taking shots at any range. That goes for all three M4 variants in the game.
While in the M4A3 75mm, you need to be within 1000-1200 yards of your target to have much effect. Only the T34/76mm has worse armor penetration capabilities beyond 1000 yards.
Use berms to help provide cover, and be sure and try to not give the enemy a perfect perpendicular shot to your front armor. Always turn slightly so the added angle helps in deflecting shots. Just remember, neither of the M4A3 Sherman tanks are stand off platforms, you dont have the armor to take the punishment.