Warp Whistle #1: On Level 3, World 1, go to near thte end of the level and find a white platform. Hold down until you fall through, then run behind the exit background. You should get one in a chest.
#2:In the World 1 dungeon, get a tail and go to the end of the first screen. Then run until the P-Meter is full and fly up out of the screen and land above the ceiling. Go to the end of the ceiling, hit up, and there's your Warp Whistle.
#3: In World 2, use a hammer in the upper-right hand corner of the world. You should destroy a rock leading to some Fire Bros. Defeat them and take your Warp Whistle.