How to use 3 external monitors on Ubuntu 17.10 Dell XPS 13 9360?
What you are trying--and which desktop you are using--is vague here. There are specific things to try, but I cannot tell what you are trying. My desktop environment is XFCE4 on Ubuntu 17.10 running the X11-based display. Not Wayland.
I have Dell Precision 5510 and a brand new USB-C dock. On the Dock itself, there are HDMI and DisplayPort jacks. I can get 3 monitors going if you count the laptop display and the 2 monitors. I have used 3 external successfully when they are plugged into separate jacks, not in a USB-C dock. I'm pretty sure you can get 2 external working via the usb-c, I suggest you try with that. Then worry about 3rd.
Here are things to try. Let us know what you see.
In terminal, run
xrandr --listmonitors
to find out if system really does see monitors. You can run xrandr to get a much more verbose listing. Right now, I'm not connected, and end of xrandr output is:
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
If you see that, then 4 monitors would be possible. Right now, I have removed the nvidia proprietary drivers and I still do have success with 2 external monitors via the dock. I expect you can too.
However, configuring will be a problem. Even if system notices your monitors, it will not use them until you configure them. You could try CLI with xrandr, but I don't do that too often anymore. It is easier if you use the GUI for this. My favorite is "arandr", which has worked great for 3 years, until last week it failed to recognize the resolutions. I have no idea what broke it.
If arandr fails, there is a much improved program called Display in the XFCE4 settings. I believe it is adapted from the Gnome project, possible your desktop has it, or similar. It lists detected monitors. You click a button "active" separately for each one, it shows them in a tiny rectangle, and you can move them about in the screen to place them left and right. If your dock is like mine, those monitors will not work until you activate.
If (IF) you are using the NVIDIA proprietary drivers for X11, run the
nvidia-settings
program. You'll see what monitors it can detect. You'll see that you cannot get all 4 monitors in one X11 session, but you can get a pairs connected with each other. You will probably not be able to drag a window across all 3 monitors. nvidia-settings will offer to re-write /etc/X11/xorg.conf for you. Make sure you have a copy of the old one before saying yes. At one time (say 2010), I was knee deep in settings for Xinerama and Nvidia Twinview. If you start bumping up against very fine grained video configurations, take a step back. It is still possible to do that stuff, but the whole push in X11 setup is to let users ignore it.
I am a little distrustful of the USB-C device you have. Unless you know for sure it works with your computer, it may be trouble. I got this computer May, 2016, and tried to order a dock. Dell said "our dock does not work, you can't buy one." I waited until Nov 2017 to get one because that was the first time the firmware was compatible with Ubuntu Linux. It mostly works now, but not always. The USB poops out about 10% of the time and I have to unplug everything. If a monitor powers down to save power, the laptop often cannot wake it up through the usb dock.
If you try and need more help, write back with more information, including what kind of dock this is, the xrandr output, and which video driver you are using, It really is a different game if you run the Nvidia proprietary driver.
Oh, if you are in Ubuntu 17.10 with Wayland as graphic environment, then I have nothing to say. I'm avoiding it.
It's possible to enable three simultaneous displays on graphics for 3rd Generation Intel® Processors and newer.
See Enabling Three Independent Displays with Intel® Graphics Controllers.
Use arandr ( Another XRandR GUI) to configure the layout. Three external monitors are working for me with Lenovo X250 (HD Graphics 5500) in Ubuntu 18.04.
Turn laptop display off and turn external monitors on (2xDP adapter to mini DP, 1xVGA).
xrandr --output VIRTUAL1 --off
xrandr --output eDP1 --off
xrandr --output DP1-3 --off
xrandr --output HDMI1 --off
xrandr --output HDMI2 --off
xrandr --output DP1-1 --primary --mode 1920x1080 --pos 1080x840 --rotate normal
xrandr --output DP1-2 --mode 1920x1080 --pos 3000x840 --rotate normal
xrandr --output DP2 --mode 1920x1080_60.00 --pos 0x0 --rotate right
Turn external monitors off and turn laptop display on
xrandr --output VIRTUAL1 --off
xrandr --output HDMI1 --off
xrandr --output HDMI2 --off
xrandr --output DP1-1 --off
xrandr --output DP1-2 --off
xrandr --output DP1-3 --off
xrandr --output DP2 --off
xrandr --output eDP1 --mode 1366x768 --pos 0x0 --rotate normal
Notes
-
If the resolution of VGA is not detected properly, configure it manually
xrandr --newmode "1920x1080_60.00" 172.80 1920 2040 2248 2576 1080 1081 1084 1118 -HSync +Vsync xrandr --addmode DP2 1920x1080_60.00
-
(Update 05/2021) 3 monitors in Ubuntu 20.04 works fine with Dell XPS 13 9565 (Intel UHD Graphics 615) and i-tec Dual Display Docking Station. displaylink-debian is needed.
shell> xrandr --listproviders
Providers: number : 5
Provider 0: id: 0x47 cap: 0x9, Source Output, Sink Offload crtcs: 3 outputs: 5 associated providers: 4 name:modesetting
Provider 1: id: 0x14e cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
Provider 2: id: 0x12c cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
Provider 3: id: 0x10a cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
Provider 4: id: 0xe8 cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 1 name:modesetting