"The Cosby Show" fell out of the Top 10 for the first time this season (its third time ever) but no one was terribly surprised. Its Thanksgiving night broadcast finished 12th. Viewing levels were down for the entire holiday weekend, but not down enough to keep an NBC schedule filled with comedy specials from coming out on top in the network race. CBS, which scored poorly with holiday specials, did well enough to catch ABC in the season-to-date averages. Both, however, trail NBC by 2.7 points--a margin unlikely to be overcome. Weekly network averages: NBC, 15.1 rating/25 share; CBS, 13.7/23; ABC, 12.4/21. Evening news ratings: CBS 11.3, ABC 10.5, NBC 10.2. (One Nielsen point equals 886,000 homes; "share" refers to the percentage of households watching a particular program.) The complete Nielsens appear on Page 10.