When you specify a JoinChannel and the channel doesn't exist, you effectively create it.
There is no way to create a channel without joining it, and it would be pointless anyway as TNet 3's packets always go to a channel. You can't send them via a broadcast anymore like in TNet 2. I suggest you reconsider what you're doing. You're trying to fit a square into a shape of a circle.
P.S. Note that you can JoinChannel without leaving your current one as TNet 3 supports multiple channels. You also don't need to specify a scene to load, so you can join a channel without anything visible happening.