World 1-3 is the 3rd level of World 1 in Super Mario Bros. 3.
Pink Note Box
Grab the red Koopa shell and kick it into the blocks. Go down to the side of the blocks and jump up. There is and invisible Magic Note Block that when jumped on leads to Coin Heaven. Or if you don't want to do it, go right.
Stand on the long white block and duck down for five seconds. You'll fall down behind the objects of the level. Run all the way to the right behind the black field. then you'll end up in a secret Toad House containing the first Magic Whistle.