I’ve owned Three Sisters, Belgrad, and Twin Peak. Comparing features and my impressions of them:
Three Sisters: Three filters with separate or mixed inputs and outputs. Can switch between triple bandpass and a lowpass/variable bandpass/highpass config. Controls are Freq and Span (adjusts the low and high bands’ distance from the center). Has negative Q, which is pretty interesting. The resonant sound of the filter can never really be dialed out. Nice for self-oscillation.
Belgrad: Two peaks, with switchable states and a single input and output. Frequency, span, and level balance control. Switchable cross-modulation between peaks. It’s flexible, has lots of variety in how it sounds with self-oscillation, but to me it felt harder to use than Three Sisters (maybe due to feeling like there were too many options selected in a discontinuous way) and the general sound for subtractive use isn’t as nice as Sisters.
Epoch Twinpeak: Morphs from a single peak lowpass to dual peak variable width bandpass (with a high Q it might as well be two bandpasses) – so it’s missing highpass unless you patch that externally via inverting and mixing with the dry signal. Two inputs (crossfaded manually), one output. There are knobs to set the two peaks separately, and each can be modulated separately, but there’s a single V/OCT input that applies to both. (Functionally, I prefer that setup because you can FM one peak while keeping the other clean and still tracking pitch.) Does not do sustained self-oscillation but pings amazingly well, and the crossfade input lets you hybridize between pinging and “traditional” use. I honestly don’t like the character of its resonance as a “normal” filter for a lot of purposes, but I do like it applied to a dubby delay.
QPAS: Obviously I don’t own it, yet Like Twinpeak the resonance never goes into sustained self-oscillation, but I like its sound character a lot better than Twinpeak. Ringing out after the input has decayed sounds gorgeous IMHO, as does the “smile pass” as a phaser. It seems a bit unfortunate that it lacks a CVable frequency offset between channels, which would be helpful for serial or independent mono, or the FM-vs-no-FM trick I like with Twinpeak.