If you complete all week 3 challenges in Fortnite season 6 you can collect the secret battle star. You can only collect the star if you completed all free and battle pass challenges. You will need a battle pass for this.
Week 3 hidden star location
This video shows you the exact location of the week 3 secret battle star.
Week 3 hidden star map
The star is located at Wailing woods on top of one of the houses:
Land on the exact location given above.
The week 3 secret star is located by the chimney on the roof of the Eastern house in Wailing Woods.
Be sure to finish all week 3 challenges, otherwise you can’t collect the week 3 hidden star. The hidden star will give you 1 free Tier for your battle pass!