[narfforc]

I decided to get out Hammond and Scull's Readers Companion and here is what it says:-

The two vast iron doors of the Black Gate under its frowning arch were fast closed.-As first published this sentence read: 'The three vast doors of the Black Gate under their frowning arches were fast closed'. It was revised in the second printing (1967) of the Allen and Unwin second Edition, to accord with the description of a single gate in Book IV, Chapter 3.

the door of The Black Gate-Prior to the Houghton Mifflin edition of 1987 and HarperCollins edition of 1994, these words read 'the middle door of the Black Gate'

I hope this helps.