New York ? So far this season, David Letterman has gotten the prime-time help he asked for from CBS.

Three of the top 15 shows in the Nielsen Media Research rankings last week were weeknight offerings aired at 9 p.m. on CBS the very time slot Letterman wanted improved when he was negotiating a new contract last spring.

There has been little immediate impact on the ratings battle between Letterman’s “Late Show” and NBC’s “Tonight” show with Jay Leno, however.

Letterman wanted CBS to work on blunting NBC’s advantage in the 9 p.m. time slots. The theory is: If more viewers are watching CBS at that hour, they’d be inclined to stay with CBS through the local news and then Letterman at 10:35 p.m.

CBS’ lone returning strong show in that hour is “Judging Amy,” which had 14.4 million viewers last week.

CBS has established the Monday spinoff, “CSI: Miami,” as a top 10 show, as many analysts had anticipated.

More unexpected is the strong showing of Thursday’s “Without a Trace,” which drew just under 16 million viewers last week despite competing against “ER” (which had 24.7 million viewers).

The new CBS shows on Wednesday and Friday, “Presidio Med” and “Robbery Homicide Division,” haven’t caught on, however.

With only two weeks of late-night ratings available, both Leno and Letterman are down 12 percent in viewers from last year.

Both shows had a ratings boost last fall as more viewers stayed up late after the Sept. 11 terrorist attacks.

Rob Burnett, “Late Show” executive producer, said Letterman’s show has seen some strong ratings in big cities on Mondays and Thursdays.

“We still have a ways to go,” Burnett said. “We’re not going to all of a sudden be the No. 1 show, but it’s encouraging.”

But NBC Entertainment President Jeff Zucker said that because there’s been no change at all in the margin between the two late-night hosts, Letterman’s theory doesn’t hold water.

“The fact of the matter is America decided between these two guys long ago,” Zucker said.