I think you will be pushing things too far... the problem is, split chargers tend not to fully charge the second battery, so I would not think that it's voltage would ever get high enough to even start to charge battery number 3.
There are 2 things you need to worry about, firstly voltage, lead acid batteries need a certain voltage to start to take a charge, this has to be higher than voltage the battery is currently at, and depending on exactly which battery type, needs to go up to around 13.8 to 14.4volts to fully charge.
Along with this voltage, you need the charging current. This is what actually tops the battery up. I do not know what alternator the Patrol has, but on the Terrano, it's alternator is just not powerful enough to cope with 2 big batteries, let alone 3.
Using arbitrary figures, if the alternator outputs 100amp, it first needs to charge the primary battery to about 80%, before it can start to charge Battery 2, this again need to get to about 80% before it starts to charge battery 3.
Physically, the cars normal running electrics use about half the alternators power, turning on things like fans, lights, and windscreen wipers can take it right up to the entire alternator output, leaving nothing to charge the battery. This is why people tend to get more flat batteries in the winter, as the car does not get a chance to replenish the battery.
But if you take the extra 50% output from the alternator, that is 50amp. The primary battery needs to replace what the glow plugs and starter took out, which can take the first 30mins of a journey. The second battery, will depend on how low it was, but in worst case scenario of it being nearly fully used, it will then take 2hours+ to charge that. The same for the third, so you would need to drive for at least 5 hours to actually charge the third, assuming you never use any headlights, wipers, or fans.
you comment about using lover voltage things on the third, I assume you actually meant lower current things, as every thing will need to be 12v rated, regardless of which battery.