I tried the Ultra SS under my VPI Classic 1 when I had it. They are tall enough to work. The only thing that hangs down under the Classic 1 is the bearing nut in the middle. Not sure if you need more clearance on the Classic 3. I did not try to thread them on. I just set them under the plinth.
I found them better than the stock feet in my system. The sound was a little cleaner and the bass was tighter/better defined. It was not a huge difference though.
See if your dealer with let you test them out before you buy.