Teams grouped together covered three miles, once (6)
I believe the answer is:
league
I believe this is a double definition.
'teams grouped together' is the first definition.
(league of sports teams)
'covered three miles once' is the second definition.
(old unit of distance equivalent to three miles)
(Other definitions for league that I've seen before include "See 11" , "Association - old measure of distance" , "sporting group" , "band together" , "Group of clubs; distance" .)