The third secret battle star location for season 5 in fortnite unlocks after you complete any 3 weekly challenge sheets. On the loading screen you get for completing the challenges you can see an ATK on the race track. The clue in this loading screen is the license plate. It points to the coordinates J2 on the map. SW stands for south-west and leads you to a dirt patch just south of the house in J2.
Simply land there and interact with the secret battle star 3 to unlock it.
The secret battle stars give you a complete tier up and unlock the next rewards in line for you.