REDIRECTION ISSUE

I think what you're looking for is something like:

when HTTP_REQUEST {
  set userhost [HTTP::host]
  if { $userhost equals "testdomain.org"} {
    set userhost "www.[HTTP::host]"
  } elseif { $userhost equals "ml.testdomain.org"} {
    set userhost "[HTTP::host]"
  } elseif { $userhost equals "testdomain.net"} {
    set userhost "www.[HTTP::host]"
  }
  HTTP::redirect "https://$userhost[HTTP::uri]"
}

Note that the way you're manipulating userhost changes depending on what it was set to coming from the client. If you see subdomain.testdomain.org, you're not going to want to prepend a www to it. If you see just testdomain.org or testdomain.net, then you will.

Make sense?

Colin