From the Super Mario Wiki
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The level features three Mini Toads and is the first level in the game not to feature any Mini Mario. The level appeared in the GDC Mario vs. Donkey Kong tech demo as Level D-3, although it was set in Clattering Cabin instead of Crumbling Cavern and featured Mini Marios instead of Mini Toads.
The course is wide and filled with Movable Pipe outlines, High Spring slots and Red Girder Rivets that help the Minis commute between platforms and sections. There are also two Color Switches that toggle Color Blocks, which are arranged to form either walls or platforms.
The Minis start on different platforms around a Movable Pipe in the middle. They must first reach a Yellow Color Switch near their their starting positions and press it to disable a column of blocks before a Blue Color Switch on the top right platform. Afterwards, they should reach and press it as well to disable a column of blocks in front of the Goal Door. The Minis then have to enter one of the level's Movable Pipes from the right, warping back to the starting section where they need to walk up a Red Girder to the Goal Door.
The M Coin is above a row of disabled Yellow Color Blocks and a pit of spikes in the bottom right. The toys must first reach and press a Yellow Color Switch to enable the blocks, then walk over them to collect the coin and avoid falling onto the spikes.