When I was learning the ropes on Slackware, there was this excellent newbie guide to installing, configuring and maintaining the distro. It has changed somewhat over the years, may not seem relevant to Debian & Co., but I still think that the SlackBook is a good piece of introductory material.
Not to pass any (imho due) judgement, but people seem to ignore the xorg manuals entirely. Maybe it's because they are made to believe that Xorg will "configure itself" as one may be accustomed with in Windows. This is not always the case and there is plenty of information out there on how to set up a dual monitor. I've answered at least two similar questions on this site about a very similar problem.
To the point then;
man xorg.conf SERVERLAYOUT SECTION (...) Screen screen-num "screen-id" position-information (...) The details are in the man page. What you want looks something like:
Section "ServerLayout" Identifier "Three monitors" Screen 0 "Screen0" 0 0 Screen 1 "Screen1" RightOf "Screen0" Screen 2 "Screen2" RightOf "Screen1" EndSection Checklist:
- Screens use correct device and monitor identifiers
- Screen has at least on Display sub section with a resolution and depth
- Drivers are loaded for the used devices
Notes:
Dual-head video cards (device section) may need special options such as bus and display mode. ATI cards used to "copy" the output to both heads in the past. This was preventable by passing certain options in the device section. Use the manual page of the xorg driver.
With recent versions of Xorg, xrandr can be used to configure and enable several monitors and to position them relative to each other. Having an xorg.config that configures several monitors is not really necessary, but may be more convenient.