Instructions
Flip into 3D while outside Merlon's house, then pass through the open gate and follow the path to get behind the house. Flip back into 2D and follow the path to the right to find a green warp pipe; enter the pipe to travel to Flipside Outskirts.
Travel left across the gaps to find a second warp pipe plugged with bricks. Use the Boomer Pixi to destroy the blocks. Travel down this pipe to enter a basement.
Walk to the right until you come to what appears to be a blank wall, then flip into 3D to reveal a narrow crack.
Use the Slim Pixi to make Mario thin enough to pass through the crack. You'll find the third Heart Pillar on the other side.