This is a really easy 15 Gamerscore, however, you do need to win the game. If you are having trouble, set the game difficulty to rookie, which should help you. Also, choose the lowest rated team or the team with the worst goaltender to make it easier to score. Then chose either the Mascots West or the Mascots East as your team and start the game. When you win the achievement pops.
In case you want more details about how to do this, here you go:
(a) NHL Threes is 3-3 on 3 hockey. In order to play this mode, look for NHL Threes on the main menu. Select it.
(b) Then select the THREES NOW mode (I chose the offline versus mode and played against the computer to unlock this).
(c) then you choose your team which has to be either the Mascots West or the Mascots East. Then choose your competition (If you want to make it easier - choose the lowest rated team or the team with the worst goaltender to make it easier to score. I chose Vancouver as my opponent - mostly so I could play against their ugly Black jerseys from the 1980s).
(d) Then start the game. If you win the achievement will POP; if you lose, rinse and repeat until it unlocks. I unlocked it on my first try.
Good luck and Happy Achievement Hunting. This is one of the easy ones.