The Debian fvwm3 has some differences from the upstream source to allow
the fvwm3 and fvwm package to be installed side by side.  This includes:

+ All the helper binaries are renamed from fvwm-<name> to fvwm3-<name>.
  This includes fvwm3-menu-desktop, fvwm3-menu-directory, fvwm3-menu-xlock,
  fvwm3-perllib, fvwm3-root, fvwm3-convert-2.6, and Fvwm3Command.  The
  Debian alternative system is then used to link them to their original
  fvwm-<name> (which are shared with the fvwm package).

+ Manual pages for modules have been renamed from FvwmModule to Fvwm3Module
  (the module names are unchanged).  mandb should set up alias for these
  when installed.  To test run `man -w fvwmpager` to see if it is linked
  to Fvwm3Pager. If not you may need to remove the `fvwm` package, update
  the mandb database to remove old entries, `sudo mandb`, then reinstall
  the fvwm3 package to get mandb to update the aliases.

+ The Debian package provides FvwmPrompt and no longer provides FvwmConsole.

If you want to install and run fvwm3 and fvwm side by side, here are some
things to consider.

+ Make sure the alternative system is linked to the helper binaries you
  want to use by default and/or call the fvwm3-name or fvwm2-name binaries
  directly.

+ fvwm3 uses the same configuration file, ~/.fvwm/config, as fvwm.  Due to
  differences in the configuration file, update the environment for fvwm3
  and fvwm to point to different locations.  This can be done by setting
  the FVWM_USERDIR environment variable and/or launching fvwm3 with the
  `-f config_file` option (the same environment variables and option work
  with fvwm).