Use the Remote section in the Scripts Editor for that and use this code:
Code:
on *:START:{
  server irc.network1.net
  server -m irc.network2.net
  server -m irc.network3.net
}

If you want to immediately join certain channels then you can add after that command, e.g.
Code:
server irc.network1.net -j #chan1,#chan2