While googling to solve my problem I came across the stack post here. However, my setting is a little different:

-----    
| D |
| V |  --------  ----------
| I |  |HDMI-1|  |Notebook|
| - |  --------  ----------
| 1 |  
-----    

The most left monitor is portrait, where the remaining screens are landscape. Is there any possibility to do the following things automatically:
Set up the above setting, while having the primary monitor HDMI-1. With primary, I mean, having the Panel there but also applications that are opened, open there (on the primary monitor).
So far I only managed to get the most left monitor to be the primary monitor (with the troubles of having every new application opened in portrait mode on the most left monitor).

I currently use Xubuntu 15.10 and xfce 4.12:

xfce4-session 4.12.1 (Xfce 4.12)

Thanks!

EDIT

Since it will most likely result in a modified randr script, this is my current randr.sh:

xrandr --output VIRTUAL1 --off --output eDP1 --mode 1600x900 --pos 3000x588 --rotate normal --output DP1 --off --output HDMI2 --off --output HDMI1 --off --output DP1-3 --off --output DP1-2 --primary --mode 1920x1080 --pos 1080x408 --rotate normal --output DP1-1 --mode 1920x1080 --pos 0x0 --rotate left --output DP2 --off

And here as a screenshot taken off arandr:

Where eDP1 is my Notebook.
Although DP1-2 is the primary screen, the panel is started on DP1-1 automatically and also new applications are opened there...
Please let me know, if there are more questions.