Video card problems on 10.13.2 - only two monitors and bad performance

I recently installed macOS 10.13.2 on my computer - hardware: https://uk.pcpartpicker.com/list/sBgJ7W - however, I haven't been able to use my internal graphics card to control my third monitor. The iGPU shows up in the System Information app, but the screen does not turn on or anything. In Linux, it does turn on. Additionally, I constantly have lagg spikes when playing games, at least in Fortnite. I haven't been able to play the game, while I installed macOS just for that purpose.