How are all three monitors connected? Are you using adapters? Are you using Eyefinity?
This link from a previous Thread at AMD Forum: R9 280x Triple Monitor not working .
Here the latest AMD Driver for your GPU Card for Windows 10 x64 : Desktop .
Your motherboard has some new BIOS and Intel Chipsets:
BIOS - F4 - 05/04/2018
INTEL CHIPSETS: Various Intel Chipsets for various functions from 03/27/2018 to 05/24/2018
If you have updated all of these then ignore the above information for your motherboard.