It's too early to tell how NBC's 179 hours of prime-time Summer Olympics coverage will score in the ratings, but the first three nights started far off the ratings record set by the 1984 games on ABC. Nevertheless, the first three nights of coverage did land in the top 10 in the Nielsens last week, the final week of the broadcast year. Sunday night's Olympics coverage was No. 2 among all prime-time shows, according to the A.C. Nielsen Co., with a rating of 17.7 and a 30 share. Saturday's coverage came in ninth, with a rating of 16.1 and a 31 share. The opening ceremonies Friday ranked 10th, with a 15.2 and 29. The rating is a percentage of the nation's estimated 88.6 million households with televisions. The share is a percentage of sets in use. The 1984 games averaged a 23.5 rating. People meters, the new viewer-measuring devices that have shown overall lower numbers for network shows, combined with a decline in network viewing may contribute to a depression in Olympics ratings this year. The first three nights of the Olympics averaged about a 30 share, somewhat lower than the third of the viewing audience NBC had predicted. NBC analyst Jeff Boehme said it was too early to tell if there's a trend in the Olympics ratings. Monday night's Olympics broadcast continued to score disappointingly in the ratings, though it won the night for NBC. The coverage during prime time rated 18.0 with a 28 share. CBS had a 14.5 for the night, and ABC a 12.9. CBS effectively programmed against the Olympics on Sunday night with a repeat of the Valerie Bertinelli movie ``Rockabye'' that ranked fourth in the Nielsens. CBS also scored with its timely ``48 Hours'' on the approach of Hurricane Gilbert, getting the highest rating yet for the weekly news program. It ranked 12th with a 14.1 rating and 24 share. NBC won the week ending Sept. 18 with an average rating of 14.0. CBS and ABC tied with 11.5 each. The week marked the 52nd since the 1987-88 fall season began, so NBC claimed its fourth broadcast-year win in a row with an average rating of 14.5. Although CBS finished third for the regular season that ended in April, based on yearlong figures, CBS edged ABC for the broadcast year with an average rating of 12.2 to ABC's 12.1. The top 10 shows last week were: ``A Different World'' NBC, ``Summer Olympics _ Sunday'' NBC, ``The Cosby Show'' NBC, ``Rockabye'' CBS, ``Monday Night Football'' ABC, ``60 Minutes'' CBS, ``Barbara Walters'' ABC, ``Murder, She Wrote'' CBS, ``Summer Olympics _ Saturday'' and ``Summer Olympics _ Opening'' NBC. The lowest-rated prime-time shows of the week were: ``Live! Dick Clark Presents,'' ``The Flintstones _ Just Say No,'' ``The `Slap' Maxwell Story,'' ``Cowboy Joe,'' ``California Girls Special,'' ``Lily Tomlin _ Sold Out,'' ``West 57th,'' ``Live from the Hard Rock Cafe,'' ``Space _ Beyond the Shuttle'' and ``Decision '88 Countdown.'' Despite a move to a half-hour earlier in New York, where its ratings dropped, the ``CBS Evening News'' rebounded into first place in the national news ratings after tying with ABC's ``World News Tonight'' the previous week. The news race last week continued to be a virtual dead heat, with less than a point separating the three network newscasts. CBS had a 10.8 rating and 22 share, ABC had 10.3 and 21, and ``NBC Nightly News'' had a rating of 10.1 and a 20 share.