Christianity.
Islam is a religon, so its the second-biggest religon in the world and the fastest-growing.
Hagerstown is the third biggest
the orca is the third biggest toothed whale
ultraosaurus was biggest the second biggest was superosaurus the third brontosaurus
North America is the third largest continent in the world. It is surpassed in size by Asia and Africa.
Callisto - a moon of Jupiter is the third largest moon in the Solar System. See related link for more information.
It is the third biggest extreme sport in the world.
India
the sperm whale
Trondheim
Aberdeen is the third biggest city in Scotland.