This is fairly easy. The grenade is the only part that makes this a bit tricky. Get to a place where there are at least 3 enemies. More enemies will make this a little easier. Make sure you have a pistol, machine gun and a grenade in your inventory. Start by killing an enemy with the pistol, then kill another with a machine gun. Finally throw a grenade at an enemy and hope you get the kill. You need to do all of this very quickly. Getting headshots will obviously make this a lot easier. If you suck at headshots then use a powerful pistol and machine gun so you can kill the enemy much quicker. The grenade will take a second or 2 to explode after you throw it so keep this in mind.
Defeat enemies with a pistol, machine gun, and grenade, in that order, in 15 seconds