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.
  FvwmPrompt is only available on amd64, arm64, i386, loong64, ppc64el,
  riscv64, and s390x. On other arches FvwmCommand can be used instead.
  To send multiple configurations, read a temporary config file:
  `Fvwm3Command Read /path/to/temporary.config`

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).

