Where are the warp whistles in Super Mario 3?

Answer 1

The first is in Level 1-3, where you must duck on the white platform until you fall through. You will then run behind the black area into the secret Warp Whistle room.

The next is in World 1-Fortress, where at the end of the level, you must fly up to the right until you fly off the screen. You then must walk to the right (you can't see your character), and enter the next Warp Whistle room.