There have been many topics regarding scripts to automatically connect you to networks when you start mIRC (and join channels).
on *:start: {
server irc.network1.net -j #chan,#chan,#chan
server -m irc.network2.net -j #chan
server -m irc.network3.net -j #chan,#chan
}
That should give you a good idea of how to do it. Just make sure -m is on all /server lines except the first. You can leave off the -j #chan part if you don't want it to join the channels automatically that way.