3 star acnh research by kaitlyn orcastraw

As long as you have the default amount of trees and flowers, and have weeds Slightly trimmed, you're fine.
After you have that, the only thing that affects 3 star progress is your furniture, bridges/ramps, and buildings.
I have no idea how much bridges or buildings affect the town, but a default town, in a speedrun, with one bridge and no museum/able sisters, needs 250 Points.
I have no knowledge of how much points you get from bridges or more shops/museum, however furniture is easy to research and easy to spam. Each furniture item gives points when placed on the ground, and it doesn't matter how it's used so long as it's outside.

The basic logic is as follows:
Small non-crafted item: 2 Points(think flower in a vase, or a cardboard box, or a chair)
Vauable non-crafted item: 3 points(the switch! it's worth 30k bells)
Basic crafted item: 3 Points(chairs, stools, birdbaths, anything that doesn't require a lot of resources)
Efforted crafted item: 4-5 Points(The apple tv requires 10 apples and gives 4 points, and the crafting table gives FIVE, so think that kinda ballpark)
Public Works Project-escue items: 7 Points(The light pole gives this many points, and I imagine the other special nook miles-only items give that same amount, it's a lot)
Fences affect points also, in a roundabout way, with DECIMALS.
50 fences = 20 points, so do the math there? idk

As long as you have 250 points, you get 3 stars.
If you want to unlock K.K. and the credits, just throw a bunch of items on the ground, that's basically it.