Yoshis Island: Super Mario Advance 3 Cheats for Game Boy Advance

Unlock Hard Mode

If you get 1000 points in a world a star will appear on the Select Game screen. Once 5 stars are earned it unlocks Hard Mode.
Unlock Secret Ending

If you get 1000 points in a world a star will appear on the Select Game screen. Once 6 stars are earned it unlocks a secret ending.
Bonus Games

At the level select screen, hold SELECT and press L, L, B, A, R to unlock bonus games.
Unlock Six Secret Levels

Beat the game and unlock 6 new levels.
Seven Lives

If you get a world score of 777 you will get 7 lives.
Spinning log boost

When at the start of the second part of Beware The Spinning Logs, jump onto a spinning log. Use the boost to help you an extreme boost for your Flutter Jump. Use the Flutter Jump to go as far to the right as possible.
Touch Fuzzy Get Dizzy: Unlimited lives

Grab a Melon Bug (the creature that rolls up into a ball) and go to the pipe with Shy Guys jumping out of it. Spit the Melon Bug close to the pipe on the left-hand side. When Shy Guys jump out, after a while you will get a "1-Up" for each time a Shy Guy hits the Melon Bug when it is in its melon form.
Mini games

Successfully complete a world with at least 700 points to unlock a mini-game:
World 1: Flip Cards
World 2: Drawing Lots
World 3: Scratch n Match
World 4: Match Cards
World 5: Roulette
World 6: Slot Machine
Easy lives

When you see a Bandit, use eggs or a portable green watermelon on him to first get three coins, then three lives.
World 0-0: Choose Yoshi color

When you start a new game and play through World 0-0 you are normally always a green Yoshi. If you want to choose your color before the level, you must play another game and complete a level on that file. If you complete World 1-1 as a green Yoshi and start a new game, your Yoshi will be pink like the one in World 1-2. If you complete World 1-7 with the red Yoshi, it will instead be blue like World 1-8.