Hello and sorry for the delay. Im still getting used to the new AMD community forums.
Does the 7870 work as intended and only the 390 is having issue? what was faulty with the R9-280?
Could be a bad card, could be some existing corrupt entries causing issues. Have you installed the driver clean without any 3rd party software running?
Tutorial for proper driver installation.
1-Load windows with only your primary graphics card plugged in.
2-Disable any and ALL anti-virus/malware protection.
3-Download and Run the 15.7 (or latest) drivers from AMD.com (Omega drivers do not have Uninstall built in, you have to do it through add/remove, which is okay)
4-Choose to Uninstall/express (may not be available in newest version, download an older version) the existing drivers, and AMD folder
5-Restart the computer.
6-Disable any and ALL anti-virus/malware protection.
7-Run the 15.7 drivers from AMD.com
8-Choose to Install/Express the drivers
9-Restart. Confirm settings are working in windows.
10-Shutdown and add 2nd/3rd cards for Xfire.
11-Respond to this ticket if you have any further questions
** If you have nvidia drivers on your computer, you might need to run the Uninstall feature of Nvidias driver as well.
Please let us know,
Mark at XFX