How can I mirror one of the screens in a 3-monitor setup?

You need to look at xrandr command

May be something like:

xrandr --output SCREEN1 --output SCREEN2 --output SCREEN3 --same-as SCREEN2

where SCREEN3 mirrors SCREEN2.

Replace SCREEN1, SCREEN2, SCREEN3 with the appropriate name that you get with the xrandr command (without any option).

For example, LDVS1, HDMI1, VGA1, etc ...

If you have an NVIDIA graphics card, you can do this rather easily in NVIDIA X Server Settings.

Under X Server Display Configuration, select a monitor and for its position, select "Same as" and then the name of your monitor, for example "ViewSonic VX2552 Series".

Fortunately for me, my monitor and TV are the same resolution!