Arena claimed the honor for the first time in 1997 while in charge of D.C. United, with his second award coming in 2009 with the Galaxy.
Under Arena, Los Angeles earned its second successive Supporters Shield this season and put together the second best regular season record (19-5-10) in the history of the league.
The Galaxy went unbeaten in 17 home games during the regular season, while also posting 17 clean sheets, which ties the record set by Kansas City in 2000.
Sunday will mark the fifth appearance in the MLS Cup final as a coach for Arena, also a league record.
Since being hired as head coach and general manager of the Galaxy on Aug. 18, 2008, Arena has compiled a record of 51-23-30 in 89 regular season games.