Units should cycle weekly (i.e. unit 1 transforms in to unit 2, unit 2 in to unit 3, unit 3 in to unit 1)
If the reason you want to cycle the units is to keep the operating hours similar between units, if you're tracking their total runtime hours, then rather than starting the "next" unit, you could start the unit with the lowest runtime. If you want the unit to be offline for a week for maintenance, then of course only check the runtime weekly.