Oh yeah, I've definitely exceeded that limit. 3 4K screens are 11520 pixels wide.
I kind of assumed it would support 3 4K screens since it has 3 displayport outputs, whereas other cards only have 2.
There is a silver lining, though: when switching the third screen to 30Hz (from 60) it actually *does* work, but it obviously doesn't look that great. Luckily, I only run a terminal window on there.
I'm guessing there are no RX480 cards that *do* support this?