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?

If yes, does it matter if 2 of the three cards are using a modded bios?

I have plenty of PSU for my task according to any of the calculators I can find online. Its a Corsair 750w.