If by 3 different sites you mean servers, that's doable, otherwise, it's not because you can have only one nick on one server.

Here's what I use for my registered nickname

Code:
on *:START: {
  server irc.server1.com
nick nick1
  server -m irc.server2.com
nick nick2
  server -m irc.server3.com
nick nick3
}

on *:NOTICE:This nickname*:*: if ($nick == NickServ) ns identify mypasswordgoeshere

on *: NOTICE:Password accepted*:*: {
  if ($network == network1) join #Channel1
  if ($network == network2) join #Channel2
  if ($network == network3) join #Channel3
}


Now, you would have to fill out some info by yourself such as irc.server1.com, nick1, network1 etc...

Copy this code, open mIRC, hit Alt + R, if window isn't empty, click File - New and paste this code. Click OK. Restart your mIRC.