Kill 3 Helghast at once using the surrounding environment

This can be tricky, most of the time the enemies aren't stupid enough to be standing on the same spot (especially not next to a barrel or whatnot). This is preferably done on easy difficulty as the ai is dumber and slower. I got this on the bridge level just before the AA gun. But basically, try and blow up all barrels, canisters and cylinders you see (tip, they are all colored red or yellow).