Range is the trump card in all tank battles. The M4A3 (W) 76mm has a cannon that is tied for all practical purposes with the T34/85mm in AP performance. The Firefly, Panther, and Tiger are all superior guns, but again the trump card is the RANGE. Be sure and compare and contrast the range charts and armor charts that are available in the hanger. Right click the M4A3 (W) in the hanger, click on "Vehicle Armor", and take notes of the penetration values at 1k and 2k, then compare those stats to the armor of the Tiger, Panther, etc.
Most tanks in AH can defeat most tanks inside 1200 yards. Beginning at 1600 yards, the tanks with the better armor protection, and that means either by raw armor or by the slope in which the armor is presented to the incoming rounds, the more armored tanks will typically last long in tank duels because they have the 9 lives to use.

Remember, the Tiger and Panther had major advantages in kill/death ratios in WWII. There are reasons for that.
None of the tanks in AH are junk. They all have something to offer and when the best attributes of each tank is used to the fullest, good things usually happen.