Grass + Fire Pledge
The combo will create a field of fire which causes small amounts of damage to your opponents each turn.
Fire + Water Pledge
The combo will create a rainbow over the battle area. This combo increases the likelihood that any secondary effects will occur when your Pokémon use moves.
Water + Grass Pledge
The combo will turn the field into a swampy moor style area which lowers the speed of your opponent's Pokémon.
If you used all three in a triple battle, the first two moves would make the combo and then the other will just be used.