Three former Genuardi's Markets in Montgomery and Bucks Counties reopened Saturday morning under the Weis brand name.   They are at Ridge and Butler Pikes near Conshohocken, Old Dublin Pike in Doylestown and West Germantown Pike in East Norriton.   The Doylestown store will be the first Weis location in Bucks County.   Weis is the 13th largest grocery store chain in the Mid-Atlantic market.   It operates 161 stores in Pennsylvania, New Jersey, Maryland, New York and West Virginia and employs about 18,000 workers.