By: Editorial Staff THE PALM BEACH Post in Florida has begun selling information to local telephone users who simply press three digits on their touch-tone phones. The premiere of Post Lines/511 represents the first non-telephone company commercial use of three-digit access granted by the Florida Public Service Commission, according to Cox Enterprises Inc., Atlanta, the Post parent company. Permission has been given for a two-year test. Services range from continuously updated news, weather, sports and stock reports to fax delivery of crossword puzzles, comic strips and tax forms. Users are charged 35? per call, which appears on regular monthly phone statements.