I have never heard a 3 cylinder locomotive, nor have I heard a recording. I have always heard "Offbeat Thunder" as a description, so that lead me to believe the drivers were quartered with the center crankpin at 45 degrees. Not so, apparently - these things are 120 degrees apart on crankpins, which would lead me to believe six evenly spaced chuffs per revolution.
The big Baldwin was quartered, but it was a compound. Nobody alive heard that one.
That makes this a wildly uneducated comment. And on re-reading, one with too much lead content.