When you're playing a multiplayer game, each character has their own specific gem that you can match in order to throw a super snowball (for example, Young Kristoff needs to match the yellow pentagon gems). The gem is shown on the screen during character select and during the game.
If you look at the snowball bar during the game, there'll be a glowing line that'll move up every time you match that specific gem. Get that line to the end of the bar 3 times in one round and you'll get the achievement!
It's easier if you just use a second controller and play against nobody, since then you won't be disrupted by any thrown snowballs.
Edit: RazorPriest pointed out the following in the comments;
"the second controller will die too early if you just spam matches, since it'll get clogged with ice from all your regular snowballs. I always end up having to switch to it and clear some ice halfway through just to keep it alive"
I personally never had this happen to me, but best to keep in mind that it is a possibility.