A group of three (4)
I believe the answer is:
trio
'a group of three' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trio that I've seen before include "Musical group" , "Bee Gees, Supremes or Andrews Sisters, say" , "Threesome" , "father, spirit and son" , "Archduke, possibly" .)