Source: https://steamcommunity.com/app/354140/discussions/0/617329797183144648/

The game Five Nights at Freddy's 3, uses a really easy to edit save method, so with some knowledge of it, we can easily hack it!

The save file is stored inside %appdata%\MMFApplications, and it's name is freddy3.

The syntax of the file is like .ini files, so it's really easy to understand and edit.

To get the game beaten with all 4 stars, you have to paste this text inside the freddy3 file.

[freddy3]
beatgame=1
beat6=1
beat7=1
level=5
bb=0
cake=0
k1=1
k2=1
k3=1
k4=1
k5=1
cine=2
goodend=1
fast=0
nocams=0
hyper=0
vents=0
turn=0
4thstar=1

Or download the file in this repository and place it inside %appdata%\MMFApplications.

Link to the file here.

This is it! When you open the game you will have all 4 stars.