Showdown will implement the Rotation battle later.
Q: Does Pokémon Showdown support Double / Triple / Rotation battles?
A: Pokémon Showdown currently supports double and triple battles. You may challenge a foe or friend to several double battles formats: VGC 2014, Smogon Doubles, Random Battle, Custom Game, or Challenge Cup. Furthermore, VGC 2014 is ladderable, as is Smogon Doubles. Pokémon Showdown still does not support Rotation battles, but there are plans to implement them at a later date, as well as Wonder Launcher battles.
Also many people suggest that in the showdown and smogon forums and it can occur later.
Nothing more to say.
Hope this helps.