ABC bumped off three ailing males - "Fish," "Baretta" and "The $6 Million Man" - and announced five new shows yesterday as it became the first network to unveil its fall programming schedule.

Just as last year, ABC had the fewest changes to announce because it finished the season the No. 1-rated network. CBS, with a large number of alterations, will announce it s schedule today in New York. NBC, in third place and certain to undergo the most elaborate overhaul of all, won't make its plans public until late next week.

ABC Entertainment President Anthony D. Thomopoulos, who helped put the new schedule together, hailed his own handiwork yesterday as "diverse, balanced and responsible."

Newtwork President James E. Duffy, said the schedule guarantees "continued leadership for the network." But the man most responsible for this year's ABC schedule and its record profits, Fred Silverman, has left ABC to become president of NBC effective early in June.

Siverman's prowess as a counterprogrammer and heightened network competition strongly suggest that all three fall network schedules will be subject to change once Silverman takes over and starts pushing buttons.

The new ABC programs announced yesterday include three situation comedies, a detective show and a sci-fi series, "Battle Star: Galactica," which already has broken records of its own for production cost of a TV series, hitting a new high of $1 million per hour (vs, an average of about $450,000). For this reason, some broadcasting insiders predict the show will be a mini-series designed to build big audences early in the season. It will air at 8 p.m. Sunday.

"Vegas," a detective series starring Robert Urich as a Las Vegas private eye, was previewed last waeek on the network as a two-hour movie and drew high ratings. Produced by Aaron Spelling Productions ("The Rookies," "Starsky and Hutch"), it will air Wednesday at 10 p.m.

"Apple Pie," at 8:30 P.m. Saturdays, is a situation comedy starring Rue McClanahan, formerly of the CBS series "Maude," as a lonely Depressionera downager who puts a family together by advertising for one in a newspaper. Norman Lear, whose T.A.T. Communications will produce the series, has compared it to his "Hot 1 Baltimore," which flopped on ABC two seasons ago, because it combines a number of wacky or desperate characters under one unlikely roof.

"Mork and Mindy," a sci-fi spoof, will star Robin Williams, a new West Coast comedian seen on NBC's revived "Laugh-In" this season, as "Mork from the planet Ork," a character introduced this year on an episoda of "Happy Days." Mork's comic adventures observing earthlings will air Mondays at 8 p.m., traditionally one of ABC's weekest weekly time periods.

"Taxi," a sit-com about a group of rowdy New York cab drivers, is being produced for ABC by a number of alumni from the revered MTM production factory, including Jim Brooks, Ed Weinberger. Stan Daniels and Dave Davis. No stars have been named for the series, which will air Tuesdays at 9:30 p.m.

Conspicuously absent from the schedule is "20/20," the new ABC news magazine show, a la CBS' "60 Minutes," which was to have been given a regular prime-time berth after a summer trial run. Executive producer Bob Shanks indicated yesterday that the program will be a monthly rather than a weekly feature in the fall. He said the scheduling of the show is one of a number of "volatile" issues yet to be resolved.

The cancellations of "Fish," "Baretta" and "$6 Million Man" were not unexpected. "Fish," starring Abe Vigoda in the character he created on "Barney Miller," bounced from time slot to time slot all season but only earned high ratings when placed right after "Happy Days." In that spot, "Look Up and Live" would be a smash.

"6 Million Man" was one of last season's hits, but it started this season with only passable ratings and declined as the season wore on. Robert Blake, the star of "Baretta," announced earlier in the season his plans to leave the program this year. It is not likely the network got down on one knee begging him to reconsider, since in recent weeks the program's share of the viewing audience has fallen below the make-or-break level of 30 percent.

ABC also said yesterday that "How the West Was Won" will not return as a weekly series until after the conclusion of the onday night football season. Then it will come back as "a series of two-hour motion pircture epics," the network crowed. Also set for next season is "Roots: The Next Generations," a 14-hour sequel to the hugely successful miniseries based on Alex Haley's "Roots."