Thanks for the input
It seems like you all have a system that works for you already. My problem is that they track in dirt and leaves (it's autumn here at the moment) and it'll only get worse when it's winter and raining all the time. I work full-time and they have the run of the house and garden while I'm gone because they are fairly well behaved in other respects.
What do you think about retraining them not to get on the couches? I've been trying to train them and my oldest male is still stubborn, but I hope he'll get it soon. The other two have more trouble getting on the couch, so maybe that's why they don't mind as much. If I get them trained, they won't be able to get on while I'm here.
And I'm also thinking of using a little gate to separate the lounge from the kitchen and backyard. That way, when I'm at work, they can have access to the kitchen and a little warm nook where I'll put their beds, but they won't be able to get to the lounge and onto the couch. What do y'all think?