Ahahah happy to give it another go!
My Three Sisters behaves as @Justmat and the manual lead you to believe—the Low output tracks a pitch CV into the Freq jack “normally” (that is, one volt per octave), while the Centre and High outputs are closeish but not exact. I’ve found that the position of the Span knob influences this.
This is just a comment on the design of the Span knob—to my understanding it just pushes the cutoff frequencies of Low and High apart from each other linearly in pitch (so exponentially in frequency … but that’s not super relevant), and the center frequency of the Centre filter splits this difference.
Practically speaking, this means that when in self-oscillation (i.e. turning the Quality knob up all the way or nearly), turning the Freq knob will move the whole chord that you would hear (at the All output, say) up and down in pitch, while turning the Span knob will change the quality of the chord, or the “notes” that make it up, if you will.
Because of that linearity that I mentioned, to my knowledge there’s nowhere on the Span knob where the output will give you a major or minor chord. There are interesting results to be found, but the only conventional chords you find are octaves.
Mannequins’ Just Friends module, on the other hand, has a knob called Intone that does have some interesting more-or-less “conventional” chords to be found on its path when using it as a sound source. One major difference between the two modules is that Just Friends is digital while Three Sisters analog, so I speculated that could be a partial explanation for the difference in behavior.
All right, this one does have a good patch note I can talk about! This is probably most easy to hear with something like the Formant output of Mangrove—something with a clear timbre you can change by twiddling knobs. So, take your Formant output and patch into Low. Probably this works best with the Mangrove at a lowish pitch, but it doesn’t matter too much.
Anyway, listen to the Low output. Span can be set however you’d like, but Quality should be up pretty high. However, not too high—the pitch from Mangrove should still be the most prominent part of the sound. Bring the Freq knob down so that it’s close to cutting off the sound but hasn’t quite started. Now, using your favorite LFO or your fingers, modulate the Freq. The modulation should be slow enough for you to hear the rise and the fall of the cutoff.
See if you hear a little ‘ringing’ that harmonizes with the pitch of the Mangrove and lightly steps up and down with the LFO. Maybe you’ll find changing the Barrel, Air and Formant settings affect the number of pitches you hear, especially as you move all the knobs toward 12 o’clock.
If you’re able to recreate this (and I hope you are, it was kind of a cool sound!) what you’re hearing is the effect of the resonant “bump” in frequency that the filter produces moving through and highlighting the various harmonics of the Mangrove’s sound. Changing the timbre of the Mangrove amounts to varying the number of harmonics and their behavior, changing the number of notes you hear “whistling” as the cutoff moves.
I’m pretty sure that this effect is not unique to either Mangrove or Three Sisters, just that it produces a more pronounced and melodious version of this effect than my other analog filter (on my MS-20 mini).
I hope this was helpful and not too long! I tried to aim at a much lower assumption of knowledge than my original comment, so I hope I was successful without veering into sounding patronizing.