How many letters does the top answer for “once in three years.” have?

The most common answer is triennially (11 letters).

Crossword clue: once in three years.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

triennially 11 letters web-def

the 300th anniversary (or the celebration of it)

once in a hundred years. 50% similar → centennially

once in four years. 50% similar → quadrennially

once in seven years. 50% similar → septennially

once in six years. 50% similar → sexennially

once in two years. 50% similar → biennially