The Giants celebrated a third World Series victory in five years with a ticker-tape parade, and it seemed everyone had a story about brought them to rainy downtown San Francisco. The city was awash in orange, black and white confetti as people packed the parade route and Civic Center Plaza to catch a glimpse of the players. --- For two teen-age girls at the parade, getting there involved a little, let's say, ''acting.'' Emily Costello, 17, and Abbey Holbrook, 16, missed class at Carlmont High School in Belmont to celebrate the victory. ''I have Giants fever,'' Holbrook said as both teens giggled.