I think it will work - might need to see if the charger will be correct for the 3 batteries with the correct voltages for the different phases of charging.
I was thinking of removing the existing 2 boxes and adding a 4 battery box. I have been using two 6 volt batteries since the DW is a 50 amp kinda girl..
I did find a duel battery box, and then I found the 4 battery box on ebay for around $70 (as I remember) that would fit the space allocated for the batteries.
I guess since I am in the 6v world - I have to work in pairs.
2015 F350 DRW, 2015 Sequoia SQ38HRL, G614's, Titan Disk Brakes, Trail-Air Flex Air pin box, SRE4000 w/ X-brace, Heavy Duty Shackle Straps w/ Wet Bolts & New Springs.. Now I am done for awhile.