The 3" measurement pertains to the sanitary fitting size, and not the flange or tube size. In this case, the flange's outer diameter will be slightly larger than 3". To understand this, place the fitting flange side down on paper and trace it. The outer diameter of a 3" sanitary fitting is 3.717". For specifics on flange and sanitary fitting dimensions, please refer to our Clamp Fittings and Gasket Sizing Guide, available on most product pages. Thanks for asking!