the holiest site in Christianity is the soul. The idea that objects places or things are to be worshiped and ranked should be considered blasphemous by any modern worshiper!
Good point. Revering a place or an object is idolatry. But since the beginnings of Christianity, the faithful have made pilgrimages. Arguably, a pilgrimage is more about enduring the journey than arriving at the destination, but pilgrimage is part of the Christian experience. A guy named Chaucer pretty much founded English literature by writing about one.
Now we're getting into questions of theology that have been hotly debated for at least the last thousand years. The subject of a library's worth of books.
My point -- it's impossible to name the "third-holiest site in Christianity," and it's similarly silly for anyone, especially outsiders, to attempt to do so for Islam.