The M3 has three turrets and from default these are all controlled at the same time, each following the mouse. This is not the best way to control your turrets. In order to use the M3 most effectively you must control each turret separately to avoid any necessary movement.

In order to do this you must switch between each turret and control them independently. Go to "Controls" and then "Tank control" then set the additional turrets to the buttons Q, E and R as seen in the image below.



Note that on the M3, the 37mm M5 cannon is considered the 'Primary weapon' while the 75mm M2 cannon is considered the "Secondary weapon". It is not necessary to bind a key for machine guns however I have it as R just in case. You will need to clear bindings for these keys and rebind anything you replaced on other keys.

You can set these to other keys if you wish however I find these to be the most easily accessible. This is important considering sometimes you may need to control the tank with WASD while switching turrets at the same time.

At the beginning of each game simply select the turret you plan to use first, you can always switch it out for a different turret later. By having only a single turret selected, only that turret will follow your mouse and only that turret will fire when you press LMB. You will also notice that this turret gets its own scope in "Sniper mode" as well.

This system allows you to keep each turret trained on a different target so while one is reloading you can be aiming and firing with the other. It can be useful for fighting multiple enemies at once without having to tediously rotate both turrets back and forth between enemies.

It is especially useful on tight maps like the town in Poland, using the 75mm and 37mm to cover different road ways makes you a force to be reckoned with.

Edit 19 April 2017: This section is now outdated as Gaijin have changed the penetration and effectiveness of all the shells. Use the shells that work best for you!

This is very simple, never use HE shells because they have low muzzle velocity and are genuinely terrible at taking out enemy tanks (excluding AA vehicles).

For the 37mm you will want to equip the M51 shot as soon as it becomes available (of course, always research Tracks, Parts and FPE first). This shot has an additional 23mm more penetrative capability than the M74 shot and you should use it on that basis as neither carry an explosive charge and both have the same ballistics
.

For the 75mm you should equip the M61 shot as soon as possible, this shot not only has an additional 5mm maximum penetrative capability than the default but also carries an explosive charge within. This charge means that often you will take out whatever you are firing at with a single shot.

With these two shells equipped, your tank will be far more effective in combat, especially against other M3 Lees while the CBT is still on(that 75mm M61 shot often takes them out in a single hit).

The M3 is an outstanding tank at BR 2.3, it has the capability to carry entire teams and hold down points from numerous enemies. The most important thing to do is angle your amour. Always try to keep the 75mm's barrel as close to the enemy as possible so that they are firing at this cross section of your tank:

As you can see, with the tank angled like this the armour plates become exceptionally thick for a battle rating of 2.3. This means that you're very unlikely to have any of your panels penetrated by anything at BR 2.3.

As you can see, the thinnest area is the plating around the turret in between the drivers hatch and the 75mm gun mantlet. I would advise you to fire directly at this spot when facing another M3 Lee. A single shot with the 75mm using the M61 shot will almost always knock out the entire crew.

The M3 is good at long range with the 37mm however it excels at close combat. Your best chance is to enter the streets in the towns in the Poland and East Europe maps. Using the 75mm and 37mm in conjunction, you can hold down multiple choke points at once. Once you have found a good spot to stay or are in a zone, the only time you will need to move is to angle your armour if an enemy comes at you from the side or rear.

The M3 does a remarkable job of soaking up hits so if you see a team mate in trouble, try to use your superior armour to shield him from incoming fire, he will be very grateful.