WORTH NOTING ON TV

* FRIDAY

ABC In Concert (ABC, 12:06-12:36 a.m.): I suppose this weekly program has a right to celebrate as it kicks off its third year. It can look back on an impressive list of guests, including The Rolling Stones, U2, The Grateful Dead, Janet Jackson, Elton John, Phil Collins, John Mellencamp, and lots of others.

The show is partying in an appropriate way - with clips from three former shows. In a red wig, Cher performs "Love and Understanding," (June, 1991). The Commitments from Ireland offer Wilson Pickett's "Mustang Sally" (September, 1991). And in a gig in December, 1992, Prince sings 'I Wanna Melt With U."

The host is bandleader Paul Shaffer from "The Late Show With David Letterman." He appears with his own band, The Party Boys of Rock 'n' Roll, in a segment taped aboard the Royal Princess Yacht while it cruised New York harbor. The songs - "Burnin' Down the House" and "Black Dog" - are from his current album, "The World's Most Dangerous Party." * SUNDAY

National Geographic Explorer (TBS, 9-11 p.m.): The season opener of cable television's longest-running documentary series - and winner of nine Emmys - features the rich marine life off the Grand Cayman Island in the Caribbean. The filmmakers attach a small, harmless camcorder - a "crittercam" - to a shark's belly to document his mysterious wanderings. (What does he actually do all day, when he isn't eating something?). They follow a loggerhead turtle as he swims among some of the stranger life forms seen

recently on TV. They even place the series' host - Robert Urich - waist-deep in stingrays at one point and picture him observing other marine life in those startlingly clear waters.

The shark experiment is part of research to preserve those creatures, reflecting the serious purpose lying behind much of this program's content.

Booknotes (C-Span, 8-9 p.m., E.T.): The guest is Harold Holtzer, author of 'The Lincoln-Douglas Debates."

Please check local listings for these programs.