You cant really use them all at same time, but you can do 2 at a time!
Grass pledge & Fire pledge
When you use the move Grass Pledge and then the move Fire Pledge during a Double or Triple battle, the move will combine and create a field of fire which causes small amounts of damage to your opponents each turn.
Fire Pledge & Water Pledge
When you use the move Fire Pledge and then the move Water Pledge during a Double or Triple battle, the move will combine and create a a rainbow over the battle area. This move increases the likelihood that any secondary effects will occur when your Pokémon use moves.
Water Pledge & Grass Pledge
When you use the move Water Pledge and then the move Grass Pledge during a Double or Triple battle, the move will combine and turn the field into a swampy moor style area which lowers the speed of your opponent's Pokémon.
Click link for more information:
http://www.serebii.net/blackwhite/comboattacks.shtml
