If you want to get it fast just scatter every item you have in your inventory around. Also, I think flowers/trees and fences help with 3 stars, so plant some of those. If you want to do it more aesthetically pleasing just decorate outside with as many items as possible and have fun with it
How many villagers do you have? I think you need at least 8. I got three stars by using a lot of fences , trees and flowers. I only had 1 incline and 3 bridges.
1. You need to have 10 villagers on your island
2. You need to build about 2 or 3 bridges and inclines
3. You need to plant a lot of flowers around your island
4. You need to make sure your island doesn't have too many weeds or have too many trees
5. You need to not have too many stuff laying around the island.