KOH

Set to 3 different clues across 3 published puzzles, most often “Soapmaking compound, chemically” — The New York Times, Saturday, 29 January 2011.

3 letters

Every clue on record, most used first

Soapmaking compound, chemicallyonce
The New York Times — Saturday, 29 January 2011
___-i-noor diamondonce
The New York Times — Sunday, 29 May 2005
Common caustic, chemicallyonce
The New York Times — Friday, 19 January 1996