The Big Bridge

The Big Bridge is a place located in the south part of the Music Box World in Wario Land 3. When Wario goes to the south part for the first time, the bridge isn't down. But by using the first Music Box found in The Grasslands, it is accessible.

Grey Chest : Miracle Seed
To get this treasure, Wario has to get the key which is located on the entrance's left. Then, he has to cross the bridge by avoiding a series of flying fishes. The treasure chest is just across the bridge.

The Miracle Seed will make vines grow in The Pool of Rain,The Grasslands and The Vast Plain. It allows Wario to open new paths.

Red Chest : Cyan Crayon
Wario needs to get the second power-up found in Tower of Revival to gain access to this treasure. To get to the chest,he only has to enter the door located deep underwater and avoid the Brr Bears to get the red key and open the red chest.

This crayon colors the sky of the hill that separates the north side from the south side.

Green Chest : Pokémon Pikachu
To get the treasure, Wario needs to pick up the red barrel located in the left side of the entrance. Then use it to break the blocks at the right side of the level. The green key is in a room that Wario can enter using an underwater door.

As for the chest, it is located by entring the door accessible after breaking the blocks with the barrel. Wario has to play a minigame of Golf and rise the minigame block which leads to the chest.

The treasure serves for no purpose.

Trivia
The Grey Chest's treasure, the Miracle Seed, bears the same name has an item in Pokémon games that is used to power up Grass-type moves.