Hello everyone. I'll keep this brief, as I've got reading to do and papers to write.
Firstly, watch this:
Now, here's the notation that reflects how I'm thinking about this exercise.
So, note that I'm always thinking of the lowest note as the 'beat.' When I practice this with two notes in the left hand and one in the right I think as the one in the right as the 'beat.'
The reason why I do it this way (as opposed to learning each permutation of voicing the beat in the three voices) is that modulation allows a greater variety of speed in rhythms. So, once I've modulated I'm free to play other rhythms that relate, in their difference in speed, to rhythms at the other two tempos.
In the video the metronome always is with the pulses five divisions apart. At a tempo of 54 b.p.m., this is the 'beat.' At the other tempos, that is a countermetrical pulse.