Forgotten fantasy childrens book about 3 races, little, big and aqua people

Question

The big people don't know the little people exist, and the little people can do a sort of magic if they know somethings true name. the plot is that theres friction between the the little people and the little aqua people, and they finally have to work together with a the big people to pump the underwater kingdom full of air since it was slowly being submerged.

Answer 1

I think it could be Power of Three by Diana Wynne Jones (description on wikipedia).