Hi Jamie,
The Generation 3 was introduced in 1990 and came after the Legend II. Before that, there was the Heritage II/Legend series. If I'm not mistaken, the reasoning behind this was that the older Kirbys like your Dual Sanitronic were labeled as the 1st-generation models, and the large machines from the Classic on were labeled as the 2nd-generation models. The G3 was basically that; Kirby's 3rd-generation machine as well as the third redesign.