Gears of War 3 may seem like a simple third-person shooter, but there's more to it. For example, running and gunning will result in death and so will camping (staying in one area) when playing online. Read below for some basic information.
Cover - In single player modes or playing against the computer, taking cover is necessary to survive. In multiplayer however, players will often move fast and will get to you quickly. In both modes, tag one side of your cover with a grenade. The computer will often get caught in the blast allowing for an easy kill. Human players may do the same, however, they may shoot your grenade. Plant your grenade somewhere hidden to avoid this.
Health - When you take damage, a red circle will appear. The more damage you take, the clearer the circle gets. Eventually, a red skull will form inside the circle. This means you have taken a lot of damage. Simply avoid taking damage to recover health.
Movement - Even though the characters you control aren't very fast, it's still key to stay moving when out of cover as much as possible. Standing still makes your an easy target. Sprinting in this game has improved over its predecessors, as you will be harder to hit while sprinting this time. Also, roll around every time you get hit.
If you're far away from cover, try to find a place where you can hide. If there's an enemy after you, shoot at him while walking backwards and rolling backwards. Sprinting back or running back makes you a very easy target due to how slow your character is.