I have the Eagle/Roadrunner to correct the speed. Once I get the new pulley, I will use the Fieckert software to compare the difference between one,two, and three belts.
I have a VPI Prime, use an Eagle/Roadrunner, currently use 3 belts with an HRX pulley, which as already explained is a necessity. I found an incremental improvement with 2 belts, but adding the 3rd belt did not make any noticeable change.