Since that film was released in the 1970’s, that was the standard surround sound configuration at that time–left(1), center(2), right(3), and 2 mono surround channels.
You can read more about here:
Dolby Digital 4.1 Surround - Alien Anthology
Edit:
I meant to say that the 4th channel is used for surround and that signal would have been sent to what ever number of rear speakers that auditorium had.