The third stage requires deeper knowedge of the game principles. It is also very beautiful level. Nice development after the second World.
Step by step solution
From the initial position, go DOWN once, in front of the first box,
push the box DOWN and DOWN.
You have freed space, to pass under the same box. Go LEFT and LEFT.
Go DOWN and DOWN.
Now go RIGHT and RIGHT again.
Push the box UP, UP, UP, UP - four times total.
Go LEFT and then UP.
Push the box to RIGHT, RIGHT and RIGHT again.
Go LEFT then LEFT again.
Go DOWN, DOWN and DOWN - three times.
Go LEFT and LEFT, then DOWN.
Push the box to the RIGHT.
Go DOWN and RIGHT.
Push the box UP, UP, UP, UP - again four times like the previous box.
Go LEFT and UP again.
Push RIGHT and RIGHT, next to the other box.
Only one box left. Go DOWN and DOWN.
You are now entering this compartment for first time. Go RIGHT, RIGHT and RIGHT.
Go DOWN and DOWN.
Go LEFT and LEFT under the last box.
Push the box UP once.
Go DOWN, RIGHT, RIGHT, UP, UP and LEFT next to the box again.
Push LEFT and LEFT.
Go UP and LEFT, over the box.
Push DOWN and DOWN.
Go around the box with LEFT, DOWN, DOWN and RIGHT.
Push the box UP, UP, UP, UP - again four times like the previous two boxes.
Go LEFT and UP for last time on this stage.
Push RIGHT.
Congratulations. You have completed the third World. Get ready for the fourth.