1) water the four potted plants. Destroy the flowers. (key one)
2) fill the water jug. Raises the gate. Put out the fire. (Key two)
3) Fill the stair container. Climb the stares. Fill the tube half way. Fill the second half. The nozzle will glow purple. Activate the nozzle. Painting guy pushes the chest out of the painting. Reducto the chest. (Key Three)
The helmet has nothing to do with the keys.
The helmet can only be done in free play with Dumbledore or Ron (Deluminator).
On the plus side if you hit the chair it will wobble then shoot up. If you jump on after it steadies but before it rises you can jump up from the chair to get something- hat or character. I can't remember which.
Hope that helped.