f you check out the php manual about the installation you'll see this in step 17:
-- begin quote --
17. Edit your httpd.conf or srm.conf file and add:
For PHP 3: AddType application/x-httpd-php3 .php3
For PHP 4: AddType application/x-httpd-php .php
You can choose any extension you wish here. .php is simply the one
we suggest.
-- end quote --
if your .php extension works, you probably have the second line in your httpd.conf.
Try adding the ".php3" extention to this line to make it look like this:
AddType application/x-httpd-php .php .php3
and restart your webserver. that should do the trick.