Here’s a handy map for finding all the Gas Stations in Fortnite from the guys at LootLake:
You can also find Gas Stations just North of Lucky Landing, East of Snobby Shores and South of Retail Row. If in doubt, remember that all Gas Stations are either in a town or on a road.
Remember that you will need to complete the match for this Challenge to register as complete. You can’t just quit out of the game once you’ve hit three Gas Stations or you’ll lose the challenge.