Groups of three
I believe the answer is:
triads
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">groups of three</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for triads that I've seen before include "Groups of sets of three" , "Chinese secret societies involved in crime" , "Eastern horse traders" , "Sets of threes" , "Threesomes" .)