Hmm... to me "tree" sounds more like "free" than "three" does. However "tree" is probably easier to hear on a scratchy radio? How about "niner" instead of "nine"? I heard it was because "nine" sounds like german "nein" meaning "no".
On the original question: the word "take off" shall ONLY be used when reading back a takeoff clearance (for you) or when issuing a takeoff clearance (for the tower). To report ready for....said event, you shall report ready for "departure".