Okay so here is my guide to get 3 stars any one level.
We will be doing this on the first level Shattered Islands.
For this one you will only need the starter pack Skylanders which come with the game.
Play up until you get to a cannon used to destroy a barrier, fire this again after destroying the barrier to destroy a house hiding a Legendary Treasure which is then obtained further on in the level.
When you reach an area called 'Market Curve' there is a Treasure Chest to the right after you see one of the townsfolk. You will find the Legendary Treasure mentioned earlier to the right of the cannon and near the key in this area.
After pushing a few turtles around you will get to a section where you push one of them out of your way to reach some jump pads, instead of using these pads, push the turtle to the left to uncover a hidden area and a Magic Gate which you can then use Spyro to access. In here you can easily collect a Hat and a Soul Gem .
Go back and use the jump pads to get on to a ledge and then head right and use this pad to find the Story Scroll.
When you get to an area called 'Chompie Pit' use gill grunt to cross the water on the left to find another Treasure Chest
You will get to an area with two gates use your sylanders to explore each of these areas.
In the water gate you will find a hat.
In the tech gate you will find another hat .
When you get to the area called 'Ancient Landmark' you will find a Treasure Chest to the right before you get the key.
And that should be all of them!
Don't forget you may need to speed run the level, skipping all possible cutscenes after doing the collectables to finish it in good enough time.