The Troika is a stationary gun usually used by Locust Drone Gunners. Once you clear the Drone off of it, you can freely use it by hitting the X Button while next to it. It is extremely effective, dealing out loads of damage to whatever it hits, beware though, the Troika has a limited field of fire and you are vulnerable from attack from behind while operating it.
Mostly present in Single Player campaign, the Troika is available in the “War Machine” level in multiplayer. -StrategyWiki