No you don't need to have multiple copies of PHP installed. I'm assuming that your files are stored as .php3 files. To get your web server to parse these files you just need to add .php3 to the line that looks like this:
AddType application/x-httpd-php .php
So that it will look like this:
AddType application/x-httpd-php .php .php3
Note this is for Apache, I don't know how to do it for other web servers.