Crossword Genius

They were wise to form a trio (4)

Ross

I believe the answer is:

magi

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'they were wise' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for magi that I've seen before include "Persian wise men" , "The Three Kings" , "Eastern men of lore; Epiphany figures" , "Three Wise Men from the East" , "Star witnesses" .)

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