First Secret Location
It can be found in the level 3-1. When you try to collect third and fourth box with health box in between. From that location go south until you reach the wall and then go west. You should be able to find a small tunnel to the south. That is the first secret location.

Second Secret Location
It can be found in the Level 4-2. Try to go South and go to West. You need to get down from the hills and travel south until you reach water.
When you see water ( Blue layer) then start going west you will be able to find the Secret .

Third Secret Location
It can be found in the level 5-2. Try to go north and east but try to be on land and you should be able to find the thrid secret.

Bonus Tip: When trying to defeat bosses there will be health cases at the corners of the map.