i have 3 monitors and the priority setting dont work. Everytime i start my pc the programs start on the other monitor.
does anyone know how to fix this?
Appears to work fine for me. However, you need to describe the desired outcome. For example under the window management setting you can set windows to open under the cursor. If you want a window to always open on the same screen the mouse is on.
I experienced a similar issue on a non-Arch Linux distro and resolved it using this method:
1- Open your terminal and run this command:
xrandr --listmonitors
The output will be something like this (I only have 2 monitors so in you case there will be a third one):
Monitors: 2
0: +*HDMI-A-1 1920/597x1080/336+1920+0 HDMI-A-1
1: +HDMI-A-2 1920/477x1080/268+0+0 HDMI-A-2
2- Now you can set your primary monitor using the appropriate output name:
xrandr --output HDMI-0 --primary
3- Add this command to your window manager’s auto-start configuration or a startup script (e.g. ~/.xprofile or ~/.xinitrc):
Hope that this help solving your issue.
Are you using x11 or wayland? Some comments are only possible on one or the other.
You are correct.
Forgot to mention that this was for X11. I switched a few months ago to Wayland when I moved from Cinnamon to KDE.
That tip was tested on X11 with Linux Mint, running on a ThinkPad T470 laptop connected to an external monitor.
this works, but i want the program to start on the primary screen or on the screen that was used last time. i now use 2 screens bc i didnt know how to set it up, now everything works fine.
i use wayland 
Are you using KDE Plasma? If so, have you tried switching to an X11 session instead of using Wayland and see if the problem persists?