Stuart , im certainty not saying dont do five parallel , who knows could be very good but some considerations might include the issues that may arise from extra airfow turbulence in the waste tank .
Also , you will not increase your lift with that method past what u have (10hg)
The extra lift is why some think the series /parallel 4vac setup gets close to a TM feel powerwise as it has both highish airflow and lift .
Other options for you ? you could put three 4.5 amp two stages one one cord and connect each to the exhausts of your motors . Should raise your suction to around 15hg .
Or use two 1400w three stages in a booster and connect to two of your motors exhausts ... your third internal vac could be switched off and as it has a one way valve it would not put a hole in the vac system when off .
You then would have the option of running three parallel or four series/parallel
something like this ...
https://www.youtube.com/watch?v=HVIBnuq50h4