Correct: the 570 does not show up in the device manager.
I can either have all three cards visible in the device manager with the infamous yellow triangle indicating they are not working properly, or I can have the 2 480s working fine and the 570 not showing up in the device manager at all.
Here is a more direct question:
Can a person run two different rx series cards on the same driver?
yes, you can run these cards, especially it is same RX series.
If yes, does it matter if 2 of the three cards are using a modded bios?
no it doesn't matter, I have 4 modded GPU's mixed with 1 stock and run smoothly.
I have plenty of PSU for my task according to any of the calculators I can find online. Its a Corsair 750w.
750w should be fine, especially when not mining, try to put the rx570 in other pci-e slot. if nothing happens, try clean uninstall,
use DDU and install blockchain driver
http://www.guru3d.com/files-details/display-driver-uninstaller-download.htmlhttp://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-Crimson-ReLive-Edition-Beta-for-Blockchain-Compute-Release-Notes.aspxhowever, the driver is for mining only and i'm not pretty sure if will decently work with gaming.