It depends what they're doing at the time.
If they're asleep or the knocking is unexpected (i.e. they haven't heard anyone coming) then the younger two will have a few really good barks. On very rare occasions the eldest might join in.
We have steps up to our door so it's quite rare for someone to come and knock on the door without the dogs hearing them coming first. They're having no problem with the kids out and about tonight as they're shrieking on their way down the road
It depends on how we react as well, if we're surprised they're more likely to bark.
We've only had one lot knock and only the whippet had a couple of barks, he has a very good bark for such an insubstantial looking dog!!
From the sound of it though some of our neighbours' dogs aren't taking the trick or treating quite so well. There's a constant chorus of distant barking at the moment.