Filed from 3 published puzzles

DPT

Set to 3 different clues across 3 published puzzles, most often “Vaccine combo” — The New York Times, Friday, 22 October 2010.

3 letters

Every clue on record, most used first

Vaccine comboonce
The New York Times — Friday, 22 October 2010
Immunization lettersonce
The New York Times — Saturday, 27 February 1999
Child's shotonce
The New York Times — Wednesday, 26 January 1994