Those are the 3 original channels that were manually panned towards the speakers.
Yes and no. The audio on subsequent mag prints is a copy of the nitrate source material, yes, but it’s a premixed version of those optical tracks, which, by themselves, had no panning information hard baked into them whatsoever. So, what you’re hearing on stereo mag prints is not a 1:1 transfer of how the audio was printed on the nitrate optical tracks, but a faithful reproduction of how they were presented in roadshow runnings of Fantasia, sans “back house” surround effects.
Just to be clear, those 3 channels are not L, C and R. They are 3 discrete arbitrary sound channels.
The mix would direct them to whatever speaker combination was desired.
Again, this was only true of the first generation nitrate optical tracks, which had no specific house “horn” assigned to them and could be manually (and later automatically) panned to any given speaker over the course of the program. But on the magnetic 35mm prints of the film, you can safely refer to the individual channels of audio as L, C, and R because, when Disney and RCA conducted the wide-band phone line transfer to save the nitrate print in 1955, they also used the only remaining Fantasound reproducer to do it, which allowed them to preserve the original dynamic range and effectively steer each optical audio track to the left, center, or right of the front stereo field as they were intended to be presented. Thus, successfully generating a final left, centre, and right channel for the 35mm stereo reissues. This is the very same mix you can hear today on the stereophonic LPs, reel-to-reel tapes and cassettes, CDs, VHSs, LaserDiscs, and the 2000 DVD release.
So, yes, they were just unassigned “arbitrary” optical tracks on the original nitrate release prints, but discrete and correctly labeled left, centre, and right channels on mag prints because of how that surviving nitrate audio was mixed, transferred, and preserved on magnetic tape in later years. Therefore, there is no need to recreate the panning effects heard in the front of the sound stage; Disney did that before the 1956 reissue. The only effects missing now would be the aforementioned “back house” surround effects which, as you’ve also mentioned, would essentially mirror “front left” to “surround right”, and vice versa, as the film plays.