That is if you don't have access to the webserver's configuration file.
If you do, simply add "php" to the addtype line for "php3".
The addhandler line is described in the php manual
AddType application/x-httpd-php3 .php3
change to
AddType application/x-httpd-php3 .php3 .php .html .shtml
or for php4
AddType application/x-httpd-php .php3 .php .html .shtml
Note: I'm not sure how well the shtml will work as shtml is also used by apache itself....