Hey, I got my three star rating today without expecting it!
I have three bridges and celebrated my first incline today.
I collected all native flowers and put them together "upstairs".
I collected foreign flowers to breed in front of my house.
I only used the free fences I got and put them around my house.
I have kept the Nook Quest furniture outside.
I have a total of about 40 furniture pieces outside, 80% crafted by me. (Also 3 outfits on display and like 20 tools on the ground.)
Today my 8 villager is preparing their house.
I have little to no weed on my island.
When I got the quest for K.K. I started placing random items I crafted. I thought it would take longer, but I was immediately at 3 stars today.
i got it with 1 incline and 2 bridges. you'll have atleast 1 bridge from the tutorial anyways, additional structures will help your rating but i don't think they're necessary if you have lots of flowers, items, fencing etc.