You might be confusing playback with recording tracks.

The 7 tracks were used for studio recording only, and then mixed down to 3 on the actual movie.
So you have 3 tracks + control (http://www.widescreenmuseum.com/sound/fantasound1.htm)
“four photocells which scan three program tracks and a pilot control-track”

The control track was there to extend the dynamic range of the audio tracks. Basically it did volume/noise control.
The manual panning was done on top of that, directing each of the 3 audio tracks towards specific speaker(s).

The 3 program tracks are not LRC, they can be any arbitrary channel combination. For example Ls, C, R or Ls, Rs, R