[cashflyer]

One off is of British origin, meaning that something is unique.
You could also have 3-off, 6-off, etc, meaning that these 3 (or however many) items are identical, but as a group are unique from all others ever made.

One of a kind means the same thing as one off.

One of three, or one of six, may or may not mean the same as 3-off or 6-off. One of three, for example, merely means one from a group of three. There is no express uniqueness in the phrase, though it may be implied from the context.

edit: nevermind... Graham's link does the trick.