Just set a server cron (if you can)
Code:
# m h dom mon dow command
*/3 * * * * wget --mirror -O/path/[newfilename].xml 'http://[source-url-path/xml/[filename].xml'
*/3 is every 3 minutes -O is a capital O
make sure the entry is all in one line and is followed by a newline \n if you paste it in SSH
$cronatb -e