I know you can battle a girl every day in Opelucid City, but in Black that is a Rotation battle (I think it's a Triple battle in White).
I've battled everyone in the game already, but is it possible to rematch anyone in a triple battle?
Rematching in Triple battles IS only found in the building in Opelucid City in WHITE. There is one in Marvelous Bridge, but it can only be battled once. So, no rematches.
You can encounter a triple battle in black. All you have to do is to go to driftveil city, then talk to the guy behind the driftveil market
HOPE I HELPED, POKEMASTER ^___^
According to this Source,There is one on the marvelous bridge.