Grants correct. The simplest way possible would require a controller that switched solenoids to do the isolation at the very least. This method is not even all that great since you would be bouncing the full system load across the batteries and they might not give a realistic value when a single battery is supporting the entire load.
You would need an algorithm to keep solenoids 1,2, and 3 open during normal operation. That would tie all 3 batteries together and the monitor would show the overall voltage.
Then maybe on a set schedule or with a manual trigger you close 3 and then 2, wait for the signal to debouce and stabilize then take a measurement that corresponds to battery 1 and record that. Then you open 2 and then close 1. Do the same measure and record battery 2. Open 3 close 2. Record battery 3, then open 1 and 2 again.
Not something that is ideal due to the voltage spikes. You could condition this some with a cap but then your ability to take a measurement would be slowed way down since you would be measuring the storage of the cap.
Now this is just giving a quick voltage read and doing it for a little longer to get ANY guess of battery run down and condition would require a bunch of more measurements since your Delta R resistance between runs will become important since you would be doing the calculations to extrapolate since you would not want extended runs on a single battery.