Well, the line would be like below, you could add it to Perform (alt-o -> Connect->Options->Perform...) for your network:
/join #channel1,#channelname,#chat
Just change the names to your liking.
But since you're using the favorites folder:
Click Edit when you have a channel selected, then check the box Join on connect and select the correct network in the combobox at the bottom... Repeat this for every channel you want to join automatically on connect.
