PHP4 or PHP5 do not have any problem to execute php3 files. The only problem is you need to force PHP4 or PHP5 to understand .php3 file extension as a php script.
Most modern Linux/BSD/UNOX system comes with PHP4 (and now PHP5). They can execute .PHP3 files without any problem.
Open you httpd.conf file and look for PHP4 configuration directive:
Once located add the following line to it:
AddType application/x-httpd-php .php3
Above line will make your server execute .php3 scripts as PHP scripts. Restart apahce to take effect:
# service httpd restart