I really struggled with this one, only to find it to be the easiest after reading this up.
Thank you so much to Twistedsymphony for this guide:http://forums.xbox-scene.com/lofiversion/index.php/t536481.h...
Quote on this achievement from the guide:
"3 pointers" are earned by making a basket while standing outside of the very large arch on the court. This achievement is for the total number of 3-pointers made by the team within a single game.
The easiest way to do this is to make sure to take control of the ball yourself as the computer won't often go for 3-point shots.
Once you've accomplished this you'll see a VIP lock icon across the bottom of the screen saying you've made 15 3-pointers in a single game. You will receive the Achievement at the end of the game.