Will power cables improve a VPI Classic 3 tt?

Can't speak for the Classic, but I have tried several different cables on my Aries 2 and I can't tell the difference. With an AC motor my sense is that as long as it is getting the voltage and the frequency needed, there would be no impact to motor performance.

Use the stock cord. It just powers the motor and has nothing to do with sound quality. A better option would be to get the VPI speed control. Those make a nice difference.

This question has been asked numerous times (on this and other forums) with regard to VPI and other tables. I'd say 80% of the respondents (myself included) hear no difference or strongly believe that it can't make a difference. Invariably, there are always the 20% who insist against all odds and reason that a power cord can influence the speed regulation of a motor or speed controller.

I agreed with Photon totally. Unless you have AC current problem in your area, defected motor or bad cable.
I also experiment with different cable to my VPI Aries 2 Extend with no success.

I use a Shunyata Diamondback Platinum ac cable on my Scout ONLY because I had one hanging around and wasn't being used. Since my entire system is wired with Shunyata including two Hydra Power Conditioners, I figured what the heck. I too hear no difference.