Favouring three letters out of nine (3)
I believe the answer is:
for
'favouring' is the definition.
(to be 'for' something is to be in favour of it)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for for that I've seen before include "Favouring" , "Pro" , "On behalf of" , "Approving" , "In support of" .)