A group of three (4)
I believe the answer is:
trio
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a group of three</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)