on *:join:#channel1,#channel2,#channel3:/whois $nick
replace #channel1,#channel2,#channel3 with your real channel names
or, if you have it inside of another script:
on *:join:#: {
if (($chan == #channel1) || ($chan == #channel2) || ($chan == #channel3)) { whois $nick }
}
or you could even do this (by filling in the variable with a channel's name separated by a comma):
on *:join:#: {
if (#chan isin %whoischannel) { whois $nick }
}
Hope that helps