Putting 3 vacs in series is a load of old cobblers, it gives a hugely dis-proportinaote lift/airflow ratio.
Nigel Woodhead has brilliantly adapted the Ashbys booster. Rather than connecting onto the exhaust, which make it a 3 vacs in-series setup, he added a hose barb to the waste tank lid so the second vac was working in parallel to the other two. This doubles the air flow. He also has adapted the main hose barb on the front of the machine to 2" which makes better use of the increased air flow.
He says it works fantastically well, and the "lop-sided" vac system apparently isn't a problem. There are photos of it somewhere.....