In this design, there are three chambers in series. First the smallest chamber resonates, then the combination of the two smallest, finally the entire volume resonates. This means that there are three separate frequencies that the box supplies the majority of the output. If the resonant frequencies are spaced correctly and the tails properly shaped, the resonant gain of the box will be spread over a larger band width than a single chamber BR. The problem is that usually there are sharp valleys between the resonant peaks. I have never been able to model a multi chamber BR that was smooth enough for my tastes. However, valleys are much less noticeable than peaks. Perhaps this particular design works. There is really only one way to tell for sure.

Bob