If you're using a web host who doesn't provide access to the httpd.conf file, or won't modify it for you, there is another option.
Look to see if you have a file called .htaccess, and make a backup of it... (just in case).
Then add the following lines:
<Files index.html>
ForceType application/x-httpd-php3
</Files>
This will force your webserver to parse all files called index.html thru php. This might not work on all flavors of UNIX...
Another, less elegant solution is to simply do a http redirect in your index page to your index.php3 page.