Yes, you can use 3 x DVI/HDMI (or mix) to power 3 screens from a 390 provided the screens are identical and connected at boot up. This is to do with the card only having 2 clock generators (required for DVI, HDMI & VGA, but not for DP) and being able to share one between two screens under these specific circumstances.
This link explains it better:
http://www.anandtech.com/show/7400/the-radeon-r9-280x-review-feat-asus-xfx/3
I'm currently running 3 screens from a 290 using DVI/DVI/HDMI and it works perfectly (my fourth runs from the motherboard's DVI as I found this better for a couple of games than running it from an Active DP-DVI adapter from the GPU).
Stu is correct, that should work if the monitors are all the same make and model.
The Radeon 390 GPU's are all able to output up to 4-5 displays each depending on the model, but this is contingent on how the monitors are connected to the card. The requirement for this is that the Graphics Cards feature either DisplayPort or Mini-DisplayPort outputs, which your Radeon has. To get 4 displays working is only possible when using either a direct DisplayPort-to-DisplayPort connection, or utilising an active DisplayPort adapter for the 3rd monitors and beyond. The Radeon 390 cards can only output two legacy/passive outputs at one time! (HDMI/DVI/VGA/passive-adapter-connected monitor) Unless as mentioned above you use three identical monitors, in which case you can output 3 displays.
Using an active adapter instead of a passive adapter utilises a small chip inside the adapter that converts the DisplayPort signal to a signal that is usable by a DVI/HDMI connection, while still maintaining the DisplayPort standard. Using a passive adapter just converts the DisplayPort output to a legacy/passive output. Active adapters are only available using the DisplayPort output, as there are currently no supported active adapters that use the DVI, VGA or HDMI outputs.
For a list of certified adapters, you can refer to
www.amd.com/dongles, as these are adapters that our certification team has tested with our cards and found them to be compatible with all AMD products. For reference regarding the difference between single-link and dual-link DVI adapters, the single-link has a maximum resolution of 1920x1200, while dual-link adapters (featuring more pins) can support up to 2560x1600. This relies on using a dual-link DVI wire as well, however.
If you are unsure how to determine if your DisplayPort adapter/cable is active or passive, any cable with Mini DisplayPort or DisplayPort on one end, and VGA/DVI/HDMI on the other end will be passive. If the adapter features an additional USB connection, or has the word "ACTIVE" physically imprinted on the adapter, then it will be active, otherwise it is most likely a passive adapter.
