One might liken the three parts to a plot progression in literature: beginning, middle, end. Perhaps a more dynamic literary realization of the three-part range would be melodic movement beginning in the middle register, moving into the low, and working its way up into the high. Such a melodic outline would be reminiscent of the Divine Comedy of Dante. It is quite possible that other hero's-journey narratives may be drawn with a three-part division of musical range. Below are a few sound samples of how the three divisions might be made with seven-string guitar and voice. I am interested to hear your comments. Thanks!
In this clip, only the open strings of my guitar are illustrated. The lowest string is round wound. The next four are flat wound. The last two are black nylon. Note the differences in tonal quality (timbre).
Another three-part division is possible on a single string. The reduced range makes unison vocal performance a more feasible task. I view the vocal division in terms of an airy, subtle, forward-placement for the low register; a full-voiced middle register; and, finally, falsetto for the upper reaches.
On one string, the open-string note represents the low register, the frets the middle, and the fretless portion of the fingerboard the upper. One does well to note that my guitar has but ten frets. The upper reaches are fretless.
NB: lower register = low Fa (open string) middle register = So La Ti Do Re (frets) high register = Mi high Fa high So (fretless)