Prefix meaning "three" (3)
I believe the answer is:
tri
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prefix meaning three</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tri that I've seen before include "Prefix indicating three" , "Prefix meaning three" , "Denotes three" , "Three-point" , "Prefix denoting three" .)