Crossword Genius

They're played in groups of three (5)

Ross

I believe the answer is:

trios

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'groups of three' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for trios that I've seen before include "Groups of three" , "Works for three players" .)

I've seen this clue in the Evening Standard.
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