You have to get 3 stars in the 6 levels of the world 3. You don't need to do the Kevin's levels.
Here is a playlist containing videos of all levels unlocked in 4 stars duo mode. They will show you the technique to use, but you just have to make 3 stars for this achievement.
https://www.youtube.com/playlist?list=PLTg8yNY-9NDMNpQs1RLGR...See also below the 6 levels separately. Good game.
Level 3-1 :
Level 3-2 :
Level 3-3 :
Level 3-4 :
Level 3-5 :
Level 3-6 :