You need to go through all the levels of the third board and do all the objectives. Here is a video made by myself showing you how to handle the different missions.
If you only need a few levels, you can watch the specific video guide below.
Level 3-1 :
Level 3-2 :
Level 3-3 :
Level 3-4 :
Level 3-5 :
Level 3-6 :
Level 3-7 :
Level 3-8 :
Level 3-9 :
Level 3-10 :
Level 3-11 :
Level 3-12 :
Level 3-13 :
Level 3-14 :
Level 3-15 :