An average of 9,543,000 viewers, equal to a 60.6% share, watched the third evening of the Sanremo Song Festival on Rai1 Thursday night, the highest third-night total since 1990.     Last year, the third evening of the festival had an average total audience of 10,700,000 viewers, equal to a 59.8% share.     With an average audience share of 60.6%, the third night of Carlo Conti's festival achieved its best result since 1990, when Sanremo, hosted by Johnny Dorelli and Gabriella Carlucci, recorded