The centre of the Emir map is a circular platform, round it there is a pool of water which has 2 dark grey cushions floating on it, these are the cushions you need to bounce on.
The easiest way to get your 3 kills is to have players line up on the arches leading to the middle of the map and then the player going for the achievement jumps on the cushion in front of the other players as it arrives and kill them in mid air, single and multikills count.
The settings I used were:
• Map - Emir
• Gamemode - My First Deathmatch
• Gamemode Settings - Score to win 3+
• Advanced Gamemode Settings - Respawn Delay 00:00
• Other settings are default
Here's a video using 2 controllers