I'm not sure what you mean about the three channels. Generally, I always thought the ".1" in 5.1, 4.1 or whatever referred to the powered subwoofer output. So, in your case, the 2.1 would be left and right mains accompanied by the subwoofer.
That depends entirely on the sound engineer. Like all formats there are good and bad recordings. I have some surround hi-rez discs where I think the center works very well and others where it is over used.
I have "multichannel" discs in 2.1, 4.0, 5.0, 5.1. Some really good "5.1" discs actually have nothing on the centre channel, so are really 4.1. You can still pin-point centre-stage; it sounds for all the world like something is coming from the centre speaker, but if you put you ear to it, there is just silence. The centre channel is almost impossible for engineers to include without messing up the phasing that give the position of sound sources so well in stereo. I think that is why many serious recording engineers give it a miss. Probably the best DVD-Audio disc I have is in 4.0. The sub also causes problems if you take a "purist" approach to music, though there is no denying its impact in movies etc.
I agree with My Rantz, it depends on the sound engineer, and what he decided to put on the disc.
John, have you listened to Eagles - Hotel California in DVD-A? IMO a masterpiece considering breakdown into multi-channel. I run my system without channel delays, but even then phase bias is negligible.