I managed to get a persistent 3 screen setup with screen 3 mirroring screen 1 and screen 2 being an extension. As far as I know there’s no graphical solution to this, but there’s a config file ~/.config/monitors.xml which controls how the physical monitors should be mapped to logical monitors. So I just had to delete one section from that file and move that tag to the tag which I wanted to be mirrored.
Copying that file into the system config directory even makes this persistent for the login screen (sudo cp ~/.config/monitors.xml /var/lib/gdm/.config/monitors.xml).
In case this is helpful to others, I’m posting my config as an example:
<monitors version="2">
<configuration>
<logicalmonitor>
<x>3840</x>
<y>0</y>
<scale>1</scale>
<monitor>
<monitorspec>
<connector>DP-2</connector>
<vendor>GSM</vendor>
<product>YOURSCREENSNAME</product>
<serial>YOURSCREENSSERIAL</serial>
</monitorspec>
<mode>
<width>3840</width>
<height>2160</height>
<rate>59.996623992919922</rate>
</mode>
</monitor>
</logicalmonitor>
<logicalmonitor>
<x>0</x>
<y>0</y>
<scale>1</scale>
<primary>yes</primary>
<monitor>
<monitorspec>
<connector>DP-1</connector>
<vendor>GSM</vendor>
<product>YOURSCREENSNAME</product>
<serial>YOURSCREENSSERIAL</serial>
</monitorspec>
<mode>
<width>3840</width>
<height>2160</height>
<rate>59.996623992919922</rate>
</mode>
</monitor>
<monitor>
<monitorspec>
<connector>DP-3</connector>
<vendor>GSM</vendor>
<product>YOURSCREENSNAME</product>
<serial>YOURSCREENSSERIAL</serial>
</monitorspec>
<mode>
<width>3840</width>
<height>2160</height>
<rate>59.996623992919922</rate>
</mode>
</monitor>
</logicalmonitor>
</configuration>
</monitors>
That configuration will produce this result (note the monitor numbering showing 2 | 4 indicating a mirrored mode):
(I have a fourth monitor connected which is disabled most of the time, hence it reads 2 | 4 instead of 2 | 1)