Chris Crawford of Baseball Prospectus asked a mix of front office personnel and scouts which minor league third baseman they would start franchise with. Unsurprisingly, the run away winner was Miguel Sano of the Minnesota Twins.
Of note, however, Ryan McMahon ranked third on three of the lists and two on two of the lists. He was either behind Sano or the Red Sox' Rafael Devers, who is currently in Single-A at age 18. In other words, in this very small sample, McMahon is rated as one of the three best third base prospects in all of baseball.