I don't have mIRC here to check the help file, but I believe you can also use -i to set the nick right on the /server line...

Code:
server irc.server1.net -i nick1
server -m irc.server2.net -i nick2
server -m irc.server3.net -i nick3


If you don't have to identify to nickserv, or if you don't mind identifying after joining the channels, you can use -j as well, which can really shorten your script.

Code:
server irc.server1.net -i nick1 -j #chan1,#chan2
server -m irc.server2.net -i nick2 -j #chan3
server -m irc.server3.net -i nick3 -j #chan4,#chan5,#chan6


Invision Support
#Invision on irc.irchighway.net