Hi,
I think this is what you are thinking, it is only a concept, not proven.
For ONE Column of THREE Rows of three digits.
To expand you could just add two more of these modules.
OR
Use the same three 595 and add the other columns in parallel, with each three digits having their own P-CH MOSFET to select which display array.
Just a thought.
Please any other ideas accepted, criticisms ignored
Tom..