Great job on level 3! You found all secrets except 2 that involved burning red(-ish) trees.
Here's some advice on what's to come:
- Before you reach level 4, you can find the following: the White Sword (buy at store), secret Moblin money of up to 150 rupees, Old Man Daniel (behind a bombable wall; I believe he's Strike's cameo character), a heart piece (at the top of the island), the blue ring (buy at store, though it's expensive and you can buy it for much cheaper later so you might wanna' wait), a bomb upgrade behind a waterfall, and a healing fairy behind a waterfall.
- You can also find another heart piece if you use the same trick that you used to find the back door to level 2. (That's before you reach Big Island.)
- If you go back to Egalos Island to buy a potion, you can find some things hidden underneath burnable trees. Look for trees that have small rocks by them.
- On the first screen of Egalos Island (where you first start after saving on the overworld), you can use an item from level 3 to trigger a secret that you'll need an item from level 4 to follow. (I hope that's not too confusing. It'll make sense once you see it.)
- I don't think level 4 is as difficult as level 3 if Darknuts are a difficulty for you (though level 4 does have a few Darknuts). Level 4 just includes Like-Likes as enemies, so if you're okay with beating them, it shouldn't be too hard. It also includes Lanmolas.
- In level 4, you'll need the Lens of Truth before you can see secret hints on the floor map; however, if you pay attention to where the item appears after you defeat the enemies in the room, that should give you a hint of a secret room (though it might be obvious when you get the map item).
Sorry to speak in riddles, but I don't want to be too much of a spoiler. Hopefully some of my advice will come in handy.
Good luck on the next island!