You will get an extra pick in your Star Chest for every level you complete with 3 stars. The more levels you complete with 3 stars, the more picks you will get when you open your Star Chest!

Star Chests are filled with rewards!
Star Chests are unlocked once you collect 20 or more total stars. Stars are collected according to the stars you receive after completing each level. Every star you have above 20 upon opening the Star Chest earns you an extra pick on the Star Chest reward board.
The reward board consists of 14 mystery prizes that you pick according to the number of picks you have when you unlocked your Star Chest.