Cobalion, Terrakion and Virizion can be found at the Pathless Plain, south of Route 131. For them to appear, you must have three Pokemon with max Evs on your team.
Each of them appear on different days:
Cobalion: Sundays, Wednesdays and Fridays
Terrakion: Tuesdays and Saturdays
Virizion: Mondays and Thursdays