I finally got around to reading this one! WooHoo! I've been following this normal form series, but I had forgotten about it until the other day and came back and re-read from the begining. Always a good read, and the subsequent discussions are always entertaining ;-)
I will say, this go has been a much more civil exchange of criticism and exposition... good job SSC Users! You've come a long way baby :-P
Like some of the other users had mentioned, I too was at first thinking "Why does this seem exactly like what was just covered in 2NF?"
After reading your responses here, it became much clearer, mainly with the A-HA! moment of "The City alone is the key in the Sales_Depot table" due to the business rule of one depot per city, which ultimaltely means one record per city. Some how that had escaped me to start and I was thinking it was not a single attribute key for some reason. Upon realizing this, it made much more sense that before it was in 2NF, and by removing the Order Number only, it moved to 3NF since the order number was not dependent on city, but only group, however the group and address are directly dependent upon ONLY the city (the key), but not each other.
Looking forward to your next entry in the series!
BTW - Why is this not a Stairway, i.e. - "Stairway to Normalization"? Seems like it should be. Maybe when it's finished? Not sure how that works