Connect 3 External Displays to iMac (Retina 5K, 27-inch, 2017)
I have attached 3 external displays to my MacBook circa late 2011. I stumbled across this nifty product:
Diamond USB Multi-Display Adapter

1 monitor is via the MiniDisplay port and 2 are attached to a USB hub (at USB 2.0 speed).

However, on some forum (that I can't even find anymore) someone mentioned this could possibly cause my CPU to overheat. I have used this 3 monitor config for a few days with no problems and I just started monitoring CPU temp.

I suspect that the performance on those USB monitors is awful? I've no idea. I know so little about graphics, I don't even know how to test them for performance...