The M3A1 is a light armoured halftrack. It is armed with a single .50 cal heavy machine gun for defense against infantry. The vehicle is used to deploy infantry troops anywhere in the combat zone.
― Heroes & Generals
Your M3 is a spawn-point for the rest of your team. Try to conceal the location. Use riverbanks, trees, hills, or other terrain masking to prevent the enemy from locating it.
Do not unlock friendly APCs. Doing so will not let your team spawn on it, and will remove the spawn capability from the APC.
Enemies love to camp these spawn-points. Be alert when spawning, and check your surroundings. If you are killed when spawning, let the rest of your team know!
If you find numerous enemies seemingly coming from a direction that is unnatural based on the map you are playing, take a friendly along and scout the area for a German SdKfz 251/1 (the German equivalent to the American M3 Half-track).
Holding crouch lets you to lower the driver's bullet proof shield.
The machine gun should only ever be used as a last effort in defending the APC, try to not use it often because it can lead to helping the enemy team discover the APC.
A good thing to use with an APC driver are grenades. This is because you can throw them in water crossings to disarm enemy mines that might be there.