I just piled a ton of furniture onto my beach and went to check the rating with Isabelle, who confirmed that throwing a bunch of furniture down had raised the rating from 2 to 3 stars. After the KK concert was organized I went back to the beach and took everything down and went about my business. Might work for you, provided you also have enough flowers - I know she can be a stickler about that.
3 star is needed and to get there you need at least 8 villagers. If you already have 2 stars (Meaning that you put on work on your island) prior to having 8 villagers, getting the total 8 will get you to 3 stars instantly.