Okay, I have 3 monitors, but prefer running my clients in 1080p on my 50" 4k tv, as I have to move my eyes and mouse shorter distances (and am more likely to notice stuff going on in other clients). I run them in fixed window mode, and then use an auto-hotkey script by Kuroro Lucifer to move them into position. It’s easy to use, uses like 300kb of RAM, and I love the results. Anyway, I use my other monitors for my web browser and chat clients, but if you’re only running 3 clients, you’ll be able to put stuff in one corner of your 4k TV (the hotkey script works with other programs, it just won’t resize them. You’ll have to do that manually).
Anyway, I can’t tell you exactly what system specs you need, but I’ll tell you what I use and the results.
I have an i7-3930k, 16GB of RAM, and a 3GB version of a GTX 1060. I can run four clients at 1080p at max settings, but will often dip below 30fps when there is a lot of stuff going on screen. So, I just turn down all my settings all the way save for shader quality, which I keep at medium. That keeps me at a rock solid 30fps, and the medium shader goes a long way towards keeping the game from looking like ass. I have done some experimentation, and feel like I have more than enough RAM and processing power for 4 clients (even at max settings). The only thing I would consider upgrading is my video card.
I consider it safe to say that you don’t need more than 16GB of RAM for three clients, but 8GB might be pushing it if you also run other stuff at the same time (i.e. a browser with multiple tabs open, TS, Discord, etcetera).
Hope that helps.