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
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.