They were wise to form a trio (4)
I believe the answer is:
magi
'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" .)