For Fred Silverman, there was only one network left to save. And he had decided to try to save it.

Silverman informed ABC Television president Frederick S. Pierce at noon Thursday that he was leaving the top programming post at the network to officer to become president and chief executive officer of NBC. Having previously served 12 years at CBS (he was hired at the age of 25), Silverman thus becomes the first person in broadcasting history to have held a top-level executive position at all three networks.

Wherever Silverman has gone, a network has prospered; he was instrumental in making once-pitiful ABC the most successful of the three. And so along with the requisite shock and surprise rattling through Manhattan canyons yesterday, there was considerable delight over at ailing NBC, which now has the No. 3 sport ABC long ago monopolized.

An official statement from parent corporation RCA president Edgar H. Griffiths hailed Silverman as "the industry's No. 1 programming executive" who could be counted on to "renew the creative vitality and sense of excitement with which NBC launched the first television network."

In a statement released by NBC, Silverman said, "I feel challenged and excited by this opportunity. It goes well beyond the scope of my present duties or any I've had in the past."

It was the limited scope of his duties at ABC that apparently caused Silverman to make the break. It is said that he felt frustrated by not being able to go higher on the corporate ladder at the network, but Pierce implied yesterday that Silverman had been offered other executive posts in the attempt to keep him at ABC.

"We'd been in discussions about Fred's continuing with ABC but not necessarily in the same position, maybe in some other capacity," Pierce said. "But his interest seemed very removed from that. I felt that he wanted to get into family entertainment at the production level."

Pierce would not detail what alternatives he'd offered Silverman, but the kind of position Silverman got at NBC was obviously not available at ABC. Griffiths specified in the announcement that Silverman would have full charge of news, sports, NBC radio and all TV network operations.

"Silverman is a man trying to probe something," said one NBC veteran. "Griffiths said specifically that he gets complete control; that must be what he insisted on."

"It's a tremendous coup for NBC," said one ABC executive. "After Robert Kintner left in 1965, they had a series of weak executives in the top spot. This guy's a real programmer and a strong executive. I think he'll give everybody fits."

Another network source said that Silverman will probably be "the highest paid executive in the history of broadcasting" - with the exception of CBS mogul William S. Paley - but no one would reveal the exact financial details of the agreement. It is expected to include large amounts of RCA stock and very large annual bonuses if Silverman can bring NBC back into the happy days.

Silverman's salary will be at least $300,000 a year, sources said. He will take over at NBC in June "at the very earliest," according to the Griffiths statement.

Pierce took pains to fend off the idea that Silverman's exit will signal a decline and fall for ABC. "I don't put any stock in that at all," he said. "Our success has been part of a long-term team effort. A lot of people were responsible for the success before Fred came, while Fred was here and will be after he leaves. I'm the one continuum; I've been here all along. So I can speak confidently.

"While we'll miss Fred, our success will continue. Our philosophy, our point of view, will remain the same. I'm the one who decided what we'd do and we'll keep doing it."

Silverman has already done planning for next year's ABC fall schedule, but Pierce said it is "highly unlikely" he will have anything more to runs until June."The programming schedule has always been my final decision," Pierce said. Pierce would not speculate on who Silverman's successor will be but indicated he will come from the current ABC executive coterie.

Pierce spoke highly of Silverman but was obviously miffed that he was joining the competition and will be programming against ABC. "Fred had indicated he'd never go to work at another network," Pierce said, "so naturally I was surprised."

Surely not everyone at NBC will be glad see Silverman arrive. NBC's chief programmer, Paul L. Klein, has been critical of Silverman's tactics in the past. In a conversation late last year, he labeled such shows as "Three's Company" to be "kid porn," called ABC's "What's Happening" "The lowest form of comedy - you can't get any lower," and complained of Silverman, "He even puts laugh tracks on the promos - laugh tracks on the promos!" (Pormos are promotional announcements plugging network shows, and Silverman is known as a master of promotion.)

Klein was at home yesterday - snowed-in like many other New York-ers - and did not respond to inquires. When reached, a spokesman said he was asleep, which at least suggested he wasn't pacing the floor.

Industry sources predicted Silverman will keep Klein on - "they talk the same language," on said - if only fear he would go to another network if fired. Reportedly less secure are such executives as netwoek president Robert Mulholland and NBC News president Les Crystal, part of the team brought aboard by Herbert S. Schlosser, the man Silverman is replacing.

The search for a Schlosser replacement has been going on for at least six months. Rumors circulated persistently in TV circles as to the remaining number of Schlosser's days and a possible successor. One insider noted, "Nobody wants the job; it's like mayor of New York."

But Silverman loves a fight -uphill ones in particular.

And RCA's Griffiths is said to be peeved that even though the television network and NBC generally are still hugely profitable, NBC is not the most profitable network and has lately fallen from a semicomfortable second place to a humiliating third.

The 40-year-old Silverman likes to refer to the competition generally as "the guy across the street." This marks the second time he has crossed the street himself. In the process he's also given a new twist to that old phrase "chance of a lifetime," since this will be his third chance of a lifetime and he's already made good on the other two.

Network reaction in New York was not especially vociferous yesterday because heavy snows kept some people at home and others, like Pierce, stranded in the office. "There are 12 people in this whole floor," groaned one NBC News employee.

In Los Angeles, eyebrows were raised to the rafters. It had been generally assumed that Silverman would stay with ABC through one more year of triumph, even though in recent weeks he acknowledged he was having talks with other corporations.

One way to confront the unthinkable is to disbelieve it. An ABC employee in Los Angeles called another to tell her of the bombshell, but it didn't take.

"Really," said the woman on the phone, "I don't think your jokes are very funny."