Two 6-volt batteries in series with one 12-volt in parallel will work but you'll probably have problems because the deep cycle 6-volt batteries will have a different charging resistance than the 12-volt battery. You may find that the charger gets confused and you'll end up with some partially charged batteries. If you're charging these while connected to shore power, why not just invest in a second battery charger?
2012 Silverado HD3500, Crew Cab, Long Bed, Dually
2013 Infinity 3850RL