The story is the waste of energy while the hot water is maintained 'at idle'.
Plainly, energy is lost every moment of the day from the hot water system. That's no problem if you're on solar power. Otherwise, the bulk of your energy consumption tends to be for the heat lost while NO WATER IS FLOWING. Tankless gains its merit from that fact.
If three phase power were available, then a single three pole C/B at 208Y120 could power up the three elements when wired delta.
Aiming at the residential market this high power insta-hot stacks three single phase 240V elements into one device.
I pity the poor installer who has to get (6) #8 conductors through a 3/4" KO. My advice: open it up to 1".
As for getting the field wiring in place start thinking of 1" PVC in the underslab/ or 3/4" ENT/SMURF tube (Flex?) with pre-installed conductors in the overhead.
Either way, quite a bump up in effort for the electrician.