You can probably use the xrandr tool for this (assuming that your driver supports the xrandr extension - I guess most do).
To check what monitors are connected and if xrandr works just type in a terminal
On my system I get for example:
Now you can configure the placement etc. of the different outputs.
xrandr --output LVDS1 --mode 1280x800 --output HDMI2 --mode 1600x1200 --left-of LVDS1 --rotate left
This configures two screen side by side, the laptop-sceen is placed on the right of the external TFT and the TFT screen is rotated by 90 degrees (portrait mode).
To switch between different external monitors, often you need to switch one off, e.g. with
xrandr --output HDMI2 --off
because a lot of graphic cards just support 2 outputs enabled at the same time.