"Destroy the AA guns (3 of 3)" is a mission objective in the game Aliens: Colonial Marines
There are a total of 3 AA Guns (Anti-Air Guns) you need to destroy
Yard with AA Guns
After destroying the second AA Gun, you will go up some ramp and through a Weyland-Yutani Medical Facility. There will be a Flamethrower Brute waiting for you there so watch out, though the brute could be busy fighting off the Xenos
There will be a Sentry Turret in this area that you can disable.
You will go through some tunnel and soon reach the last AA Gun, when you enter the area of the gun there will be a shack to your right with lots of weapons including the M5 RPG needed to destroy the AA Gun.