How do I set up a login script, to run xrandr?

I'm using a 2 screens layout which is a bit similar to yours, mine is a regular screen on the right and a portrait-oriented one on the left. With my setup I have X working perfectly on my 2 screens.

Here's my proposition for your own case (hard to test as I don't have the same screens and don't have 3 screens) but that should be enough for you to get a working X setup.

Put the following files in /etc/X11/xorg.conf.d/

30-screen-dport0.conf
30-screen-dport1.conf
30-screen-dport2.conf

with the following content :

30-screen-dportcenter.conf

Section "Monitor"
  Identifier   "DisplayPort-0"
  Option       "Primary" "true"
  Option       "PreferredMode"   "3840x2160" # Adapt this if you resolution is not the same
  Option       "Position"        "1200 0" 
EndSection

Section "Screen"
  Identifier   "DPC"
  Device       "nVidia" # here you choose your driver
  Monitor      "DisplayPort-0"
EndSection

30-screen-dportleft.conf

Section "Monitor"
  Identifier   "DisplayPort-1 "
  Option       "LeftOf" "DisplayPort-0"
  Option       "Rotate" "left" 
  Option       "PreferredMode"   "1920x1200"
  Option       "Position"        "0 0"
EndSection

Section "Screen"
  Identifier   "DPL"
  Device       "nVidia"
  Monitor      "DisplayPort-1"
EndSection

30-screen-dportright.conf

Section "Monitor"
  Identifier   "DisplayPort-2"
  Option       "RightOf" "DisplayPort-0"
  Option       "Rotate" "right" 
  Option       "PreferredMode"   "1920x1200"
  Option       "Position"        "5040 0" # 1200 + 3840
EndSection

Section "Screen"
  Identifier   "DPR"
  Device       "nVidia"
  Monitor      "DisplayPort-2"
EndSection

90-serverlayout.conf

Section "ServerLayout"
    Identifier   "Main"
    Screen       0 "DPL"
    Screen       1 "DPC"
    Screen       2 "DPR
EndSection

---

The coordinates of the Xserver works the following way

 0                 X
+ -----------------> X-axis 
|0
|
|
|
|
| 
| Y
V Y-axis  

The nVidia identifier is a reference to a video card defined in a file called

20-nvidia.conf

Section "Device"
  Identifier  "nVidia"
  Driver      "nouveau"
  Option      "AccelMethod"  "sna"
  Option      "GLXVBlank"    "true"
  # Need to flag this as only referring to one output on the card
  Screen      0

EndSection

I use a user-land systemd service (full path = $HOME/.config/systemd/user/set-display.service) to handle a similar situation on my QEMU VMs that use XFCE and do not automatically fill the screen (cut short due to conky):

[Unit]
Description=Set Display Resolution

[Service]
ExecStartPre=/bin/sleep 5
Type=oneshot
ExecStart=/usr/bin/xrandr --output Virtual-0 --mode 1499x996

[Install]
WantedBy=default.target

I use the sleep command to make sure that the DE is fully loaded; my VMs on SSDs were always fine with 5 seconds or less, the HDD ones sometimes needed more. Just create a script that runs both of your xrandr commands, substitute it on the ExecStart= (something like ExecStart=/path/to/your/script), then enable it (systemctl --user enable set-display).

UPDATE: testing as system service on Debian Buster did not work, even when running the service manually after logging in (tried as root and normal user).