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
The second location can be found on the level 4-2. You need to travel down the hill in south direction. When reach the bottom of hill you will find water and then travel to your west until you reach water on west and south, you will find the secret.
Third Secret Location
The third location is on level 5-2. You can find the location by travelling to north and east from where you spawn. Do not go into the water and travel only on land you will be able to find the location after a couple of houses.