The M3 is a very powerful pump-action shotgun available to both teams. The M3 is very effective in close quarters combat and is commonly seen on Office and Nuke. Because both shotguns are fairly similar, both are used. For those reasons, the M3 is a fairly popular weapon although not comparable to the AWSM, M4A1, AK-47, XM1014, or Desert Eagle and is almost never seen in tournaments and in public servers.
The M3 is a fairly heavy weapon. While wielded, the player's movement speed is 220 units per second (default speed is 250), meaning they suffer heavy speed reduction. The M3 is one of the few weapons that is unable to fire underwater.
Aiming for the head is the best technique to obtain easy one hit kills at close quarters. However, it can be risky as some of the pellets may miss your target so be sure to confront the enemy within acceptable distances and when no one is aware of your presence.
Take cover while reloading, also, try to take cover or strafe between each pump.
Combining this weapon with the use of grenades and/or fast pistols is a good strategy.
If hiding in corners, this weapon and the Benelli M4 Super 90 are the best for easy and quick kills.
Try not to be out in the open with this weapon equiped. Players can easily kill you at long range.
This may be a shotgun but do not underestimate its range. Shotgun pellets do not disappear after a certain distance in Valve games so this weapon has a longer range than would be expected in an FPS.
Getting a second shot is unlikely against rifle or SMG users. When possible, don't fire until you can expect 100 damage.
Work with your teammates, if your shot doesn't kill, your teammates can take the oppoent down, or you can finish up wounded enemies at close range.
When facing an enemy close to your position, ensure that you fire your M3 first, perfectably at the head or chest, and retreat when you have to pump the weapon after firing a shell. You can also use a flashbang with this tactic to assist you.
Shooting while hiding around a corner can increase your chances of scoring a kill.
Use long and medium range weapons against its users.
Grenades against its users also works, flash its users if you need to fight in close quarters.
Avoid close contact with M3 users.
If you have Sniper rifle and the M3 user is close, keep moving and use your sidearm to defend yourself.
Automatic weapons may give the user an advantage due to the M3's slow rate of fire.
The pumping animation for the M3 can hamper the sight perspective for users thus making them unable to clearly see where their targets are moving. Due to this, use this exploit to your advantage by strafe left-right and gunning down the M3 user.
If an M3 user misses you and you have an auto weapon, they're doomed.
In-game, the M3 does not have the ability to switch from pump action to semi-automatic operation (which can be done with the real life counterpart).
The M3 does more damage than M1014, despite they have same muzzle energy, in Deleted Scenes however, the effect is the same.
In Counter-Strike: Source, the view model shows that the rear sights have been removed, but the sights still show up on the weapon's world model. This shows that the view model is remade after BETA stage.
The M3 is the only weapon that can perform an instant kill if hit the legs, possibly to balance the pumping action, in case if all pellets hit the target.