Whats the third element?

The third is technically the Water Element because you have to wait for the Palace of Winds to get the Wind Element. To get the Water Element after defeating Mazaal in the Fortress of Winds, you have to first talk to one of the Minish in the upper left bookshelf on the second floor, then talk to the left librarian. She'll tell you that three books are missing, and it's your job to claim all three. After yo do that meet with Librari in the book with the clover on it. He activate a secret panel which sends you to a small cavern with some slugs to kill. You'll eventually find a door which will trap you with two Scissors Beetles once you enter. Kill them both and collect the Flippers, a vital item since you have to swim to even have a chance at the Water Element...speaking of which, you'll find it in the Temple of Droplets, the small iceberg like structure in the left center of Lake Hylia. Just stand on the crack and shrink to get in, and you're on your way.