The card is Gigabyte R9 280 Windforce3. After upgrading latest drivers to version 18.7.1 I've lost Eyefinity at all. It says something like 'This system does not support AMD Eyefinity.'.
Whatever. The main problem is triple screen setup, even under windows 10. Do I really need to play with some pseudo-active adapters to make it work? I have two adapters for DP->HDMI, third screen is connected via hdmi or dvi. I'm not sure whether they are active/passive, but... but why it works on Ubuntu 18? I barely started the installer for testing purposes and I've noticed third screen got up and running. After the install I had just to reorder screens to match and it works out of the box. So... WTF?