This is a quick guide about the location of the 3 hidden cows.

The first one is located in Forest 8, the cow is pretty easy to find.
The second cow is located in caverns 7, It's a bit tricky to find it. You need to access to the middle of the map via the secret passage on the right hidden by a crate.
The third and last cow is located in Hive 5, the cow is waiting on top of the map to get there you need to do some platforming with the huge bumblebee.