The week 3 star in Fortnite season 9 is located in Junk junction. You will find a pile of trucks and an ice cream truck in front of Junk Junction. The secret battle star is located on top of that pile. You can only collect the secret battle star for week 3 if you have completed all challenges and have unlocked the week 3 loading screen. You also need a battle pass.
The week 3 loading screen gives the hint of the secret star location.
We show you the location of the week 3 battle star in this short video: