3.5nm is a pipe dream so remote this is the first time I've heard anyone even mention it. 7nm is a pipe dream at least two years away. Even 10nm for practical/reliable use is at least a year away. Remember there are only about two places on the planet one can order up 14/16nm designs, and only about two places on the planet currently even attempting 10nm fabrication.

If you check out Mining Speculation, you'll find more discussion on how foolish it is to expect anyone to put up 7nm miners in any kind of timeframe to affect current utility of S9s.