Our trailer is too small for a 3 battery setup (and it wouldn't be necessary anyway.) We are already on LEDs (came with them from the factory) so the biggest power draw is the 2 way fridge (which we rarely use in any case.)
The reason I'm going to the 2 x 6v setup is that our current 12v battery is shot and rather than replace it with another 12v/78ah battery, it makes more sense to just go ahead and do the dual 6v conversion (which gives me 230ah) as many others with our type of trailer (T@B) have done.