Beautiful patch. Can’t believe I never thought of trying this, thank you for your insight! So glad I have two of these to take advantage of all that it can do 
Recently I’ve been using Three Sisters to filter the delay line of my DLD, with the help of the tanh3 from Instruo which prevents crazy feedback/adds grit. I’ve found that using the center filter in crossover mode to be the most flexible for said purpose and can lead to some very BBD-esque tones. It’s so much fun to ride the respective feedback controls and adjust the filter frequency to emphasize harmonics.
I also really like using it in a drone patch like so: formant mode, full CCW on quality, multiple soundsources in, mix out. I think this creates some sort of notch filtering? It’s hard to describe what it sounds like other than pushing and pulling the sound sources around without obvious resonance… whatever the case may be way, swaying the frequency knob is so satisfying in this set-up.
Also also really like using it as an oscillator even though tuning it precisely is a huge pain… I use the 2 operator FM patch from the manual a lot but with the filters almost in unison. Adds a subtle thickening that’s quite pleasing.
I love this module. Not always my favorite filter sounds but its design is so flexible and inspiring.