Track Packs in Bloons Tower Defense 3 are groups of 3 tracks which, when beaten, awards the player with a Bronze Medal (Easy Difficulty), Silver Medal (Medium Difficulty), or Gold Medal (Hard Difficulty).
The Classic 1 Pack and the Racing Pack are initially unlocked. The Beach Pack is unlocked by beating 3 tracks. The Snow Pack is unlocked by beating 6 tracks. And the Classic 2 Pack is unlocked by beating 9 tracks. Any unlocked track can be beat to count towards unlocking the next track pack.