This DVD set contains all of season three episodes, including the Christmas episode from Season 3, which was not included in the U.S. (Region 1) version. You do need to use a Region-free player (either hardware or software) to watch these, if you are not in Great Britain, but that was no problem. Quality is excellent, as expected, since these are produced by the studio.
If you don't own Season 3 yet, I would highly recommend that you purchase this set, rather than the Region 1, so you can get the Christmas episode. Most people who seriously collect DVDs usually have a region-free DVD player anyway and if you don't, you can purchase one quite cheaply. Bing it.