I think it is clear you only count the phases, the neutrals will only carry the unbalanced current. That puts you in the 70% derating column. That will give you the 60+ amps required for 60a breakers and not kick the 310.15(B)(6) tar baby.
I would certainly be troubled if someone wanted to say this was good for 100a but the expected diversity would allow me to sleep well at 60a per.
