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You have installed the mod_php3 package.

Have a look at the mod_php3 _port_ if you need additional support for
databases other than MySQL, e. g.

You can find it at
ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/ports/www/mod_php3.tar

Make sure the mime.types are connected to the php module properly and
index.php3 is part of your DirectoryIndex.

The following should be in your Apache configuration file:
[...]
DirectoryIndex index.php3 index.html
[...]
AddType application/x-httpd-php3 .php3
AddType application/x-httpd-php3-source .php3s
[...]
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