> host, who knows what’s behind the doors, opens another door, say #3, which has a goat.

There is quite a lot of ambiguitiy in how this is phrased. So the host knows what is behind the doors, but how does this affect his choice to open another door?

Does he always open another door, or does it depend on what is behind the door the contestant selected?

Is the second door selected by random or will it always be one with a goat?

The implicit rule is that the host: 1) always opens one of the other two doors, regardless of what is behind the selected door. 2) deliberately opens one of the two doors which doest have a car behind it.