I am replying to this old thread because I'm using this exact code.
When I try to put the join channel command here it loads two instances of the same server instead of joining a channel on each server list.
I think I'm doing something wrong here.
Use the Remote section in the Scripts Editor for that and use this 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.
server irc.network1.net -j #chan1,#chan2