The card that works fine will work fine by itself or when in a rig with other cards.
The cards that don't work as expected behave the same if they're in a rig by themselves or with other cards.
The same cards fail regardless if I specify the voltages together or individually (eg. -cvddc 800  or -cvddc 800,800,800).

I have reinstalled drivers, reset all the settings for all cards in radeon software befopre running the miner, etc etc.
It feels almost like 2 of the 3 cards have some kind of different hardware on them that regulates voltage and isn't very well supported by phoenix, but that seems pretty unlikely given they're all the same brand and it's a new card...