三枚
さんまい three flat objects (sheets of paper, pieces of cloth, etc.)
Why it means this
三 (three) + 枚 (a counter for flat thin things) — three (sheets, plates); 三枚におろす, to fillet a fish into three; 三枚目, the comic-relief character.
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.