First, register with AuthServ if you have not already.
Then, it might help to skim our modes help file.
You have a few options:
“s” and “p” make the channel secret (not shown) and private (masked out) so that no-one knows the channels exist.
Channel passwords are handled by the channel mode “k”.
Basically you would issue:
/mode #yourchannel +k thepassword
And then to join the channel after that:
/join #yourchannel thepassword
You can make the password (and other modes) more perminant by telling x3 to always set it:
.set modes +ntk thepassword
If you want in, but cant remember the password:
/msg x3 #yourchannel inviteme
you can use invites to let people in without telling them the password (so, for example you can add them as peon or op to x3, and they can use the '/msg x3 #yourchannel inviteme' command to get in – or better yet they can set '.uset autoinvite 1' to have x3 invite them automatically when they are logged in.
You could also use channel mode +i (instead of +k), so there would be no joining unless they had an invite (from x3, inviteme, or another command)
If you change your mind, you can remove any mode by typing: '/mode #yourchannel -k thepassword' or '/mode #yourchannel -i' etc. Also, x3 can remove all locking modes:
/msg x3 open #yourchannel