The easiest way to do this, is to use the mlock command that most chanserv's have. For more help look at /cs help mlock or /cs help set mlock...different chanservs have it in different locations.

However, if this option isn't available or for some reason you don't want to do it that way then you might try
Code:
on @*:mode:#thethirdchannel:{ .mode $chan $+(-,$1,+nt) }