There are only three keys, one in the second part of each level; One in Forest 2, one in Cave 2 and one in Mine 2. They're usually off the main path to the exit, so you have to look hard for them.

one is in forest 2, one is in caves 3, and the last is in mines 2
the keys are on the last area of the level
forest has 2 areas, caves has 3 areas, mines has 2 areas, and ruins has 3 areas

All three keys are in the forest2,cave2 & mine2, A simple thing to know right?XD

They are hidden in:
FOREST 2
CAVE 2
MINE 2

Search all rooms for them. Good luck.