Hi GB , I have just done a rebuild and to play with triples at 1080p was an after thought.
I bought the Following.
Amd Ryzen 2600 (not overclocked but turbo boost to 4.2 ghz , I got lucky on the silicon lottery)
Radeon rx 580 8gb
NV me Drive 500 gig ( Only os is installed on this drive and is 5 times faster than an SSD)
16 gig 3200mhz ram ddr4
Asus prime b450m motherboard
3 x Lenovo Thinkvision 27inch 1920x 1080 75hz 4 Ms screens
All works well in single player on high settings , CPU runs at 50% Gpu 98% and get around 90fps on average.
In an online race the fps drops as low as 45 fps and I really notice the stutters when it does so, which effects the immersion and also the input response times. The third last turn at Okayama was horrid for some reason.
The monitors are free sync enabled ( to go with the Amd GPU ) .
I have had a lot of trouble with refresh rates not matching on all 3 monitors and getting a lot of flickering. After 1 month of research the refresh rate flickering issues is down to AMD eyefinity ( the equivalent Nvidia's surround) is not able to cope with using different Gpu outputs. i.e. using 2 HDMI outputs and 1 Display port output or the dvi-d one. Eyefinity needs to use, from what I have read and experienced is the one type of output to be able to match refresh rate across all three screens.
I have managed to fix this issue by not using eyefinity and simply letting windows handle triple screen by extending the desktop and running in windowed borderless mode , which required a few extra settings in the app.ini file and works well now.
Now I also run a 4th screen in a different room and Assetto Corsa seems to handle this ok as long as I make sure in the display settings that the 4th screen is above the three screens. I never have the 4th screen turned on when playing with the triple screens fyi.
The other problems I have had is that I also have iracing and Project Cars 2 and to use them in triple screens with out using eyefinity that 4th screen has to be disconnected physically.
If I had my time again I would make sure that the GPU had 3 outputs of the same type , ideally Display port as they are the best to use ( from what I have researched is that display ports are the only one capable to do 4k gaming above 60hz ) but also ideally make sure your screens also have display port inputs. For me personally I find 75 hz refresh pretty good I can noticed the difference to when I had it set to 60.
I'd also buy a better GPU, the Gpu I bought was best bang for buck but I would like to be able to run at least 75 fps all the time I am thinking of dropping my resolution down to 1600 x 900 for online racing as I don't really notice the difference and it is 25% less pixels the GPU needs to draw. A minimum of 75 fps keeps the image nice and smooth and matches the monitors capabilities. I haven't tested this yet to see if I get a minimum of 75fps.
For 1440 resolution your going to need a top end GPU I envisage .
I haven't tested my CPU usage in an online race so can't comment on that, I did however notice that if I turned of all unneeded apps in Assetto Corsa the FPS rose, suggesting to me that the CPU was being stretched to its limits.
AMD is supposedly releasing Intel and Nvidia killing CPU and GPU's on July 7th so a few months after that the price for things will drop a lot from where it is now on bang for buck level.
As an FYI my current system with out the Hard drives, PSU and Case cost about $1800 , $660 of which was for the screens just give you an idea of the cost.