From the Super Mario Wiki
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3-2-1 Blocks are a special type of block only appearing in the VS Game and You VS Boo modes in Super Mario Bros. Deluxe. The blocks start out as white and count down from three to one, at which point they change from white to red and the countdown restarts. In their first appearance in stage 1-3 they change Dotted Line Blocks into solid blocks and back, while in stage 2-2 and beyond they change solid blocks into Spiked Blocks. Hitting the blocks causes them to automatically change, the countdown resetting as well.