I had a similar problem the last days. I have an Alienware 15 R3 with GTX 1070. Using Ubuntu 18.04 (LTS... kernel 4.15, I think) didn’t work, also the latest Manjaro (kernel 4.19) didn’t. Using Ubuntu 18.10 (Kernel 4.18) I was able to hook up 2x 4Ks (1x USB-C to HDMI and 1x Mini-DP to Display Port) and 1x QHD (1440p) via HDMI; I was also able to use the laptop screen (1080p). I think only the Mini-DP to DP is outputting at 60 Hz, the other 2 screens at 30 Hz. I think turning off Thunderbolt 3 on the USB-C port was possible in Ubuntu 18.10 which made the difference (I also read in a forum that this made a difference for someone else; but I couldn’t turn off Thunderbolt 3 in my BIOS).

I think in the latest kernel developments there have been lots of updates on USB-C and ports in general.

But that’s why I like Linux as well; it gives me the options to use different distributions and environments. It might be messy sometimes, but I’m not locked in to one vendor.

I've set this up before. How many video cards are you trying to use to connect these monitors? I've always had it just work if you are only using one card. In the past I've had to invoke xrandr with the --setprovideroutputsource flag (or was it --setprovideroffloadsink ?) to get multiple monitors to work with multiple video cards. Note, this only works with the open source drivers and you must be using an X server that supports randr 1.4.