With only three first-round draft choices having signed with National Football League teams, the league is facing an unprecedented number of rookie holdouts as most of the 28 teams open training camp this week.

As of late yesterday, only 138 of the 333 players selected in the league draft had signed, according to the NFL Players Association, including only 11 of the 84 selected in the first three rounds. The only first-round picks signed are Atlanta nose tackle Tony Casillas (taken second overall), Tampa Bay defensive back Rod Jones and Raiders defensive end Bob Buczkowski.

The Washington Redskins, who will open camp Monday, have signed only two of 11 picks, their ninth- and 11th-round choices. The Dallas Cowboys opened camp 12 days ago and have signed all but three of their picks -- the top three. The New England Patriots will begin Friday and have signed only three of their 12 picks -- a seventh-rounder, a 10th-rounder and an 11th-rounder. The Chicago Bears are in Platteville, Wis., and have signed all but their top two choices.

The signing pace for middle- and late-round picks is slightly behind last year's pace; that for the top three rounds isn't even close.

Of the 321 NFL veterans who played out their contracts last season and officially became free agents Feb. 1, "about 100" have signed, according to Jim Miller of the NFL Management Council.

Last year, NFL players earned an average salary of about $200,000, more than double the 1981 average ($90,000), but less than half the current average salary of major league baseball players (more than $400,000).

Player agents and the NFLPA say that, with minimal competition from the U.S. Football League, clubs now are trying to roll back offers to 1983 levels. Signing bonuses were slashed last year, and this year's salary offers are being held at last year's levels or are being reduced, agents say.

The length of contracts also is being hotly contested now. Because the NFLPA will seek free agency as its No. 1 priority when the current collective bargaining agreement expires after this season, NFL teams want players to sign longer contracts, in part to keep them from becoming free agents in the short term.

"It's the only business I know with a healthy economy that attempts to slash its employees' salaries," said Leigh Steinberg, who represents two as-yet-unsigned No. 1 picks, San Diego's James FitzPatrick and Dallas' Mike Sherrard. "I see this as the Spanish-American War on this year's rookies to next year's Civil War, where the NFL is using its muscle as a warning to intimidate players as collective bargaining approaches, saying 'Look here.' "

George Young, the Giants' general manager, said, "Who are these agents, Albert Schweitzers? Next time they plead about how they are freeing the slaves, put a picture of their cars and houses next to the story. . . . We're escalating salaries by over 20 percent a year across the board , and we can't keep committing this hara-kiri. We're not trying to get these guys for the lowest price possible; we're trying to create a sensible market."

Saints General Manager Jim Finks, who has signed only three of 11 draft picks, said, "Clubs are scared to death by what they've done to themselves over the past three years. They have heard enough saber-rattling from the networks, and their message is loud and clear and confirmed by Pete Rozelle, league commissioner : Don't expect a big jump when we negotiate a new TV deal next year. And the players union has gone on record saying free agency could be a strike issue in 1987 .

"Now we're seeing a real concerted effort by the union and the agents to push clubs to the wall. They'll use every tactic available, and the one they are using now is holding their clients out and waiting to see who'll blink first."

Contradictory numbers are being tossed out. Young said, "If we continue to accelerate at the rate we're going, we will have to get a TV contract for $3.7 billion over five years next year." The current deal is $2.1 billion for five years.

Jack Donlan, executive director of the NFL Management Council, said the league will receive $493 million from the networks this year, including the postseason. Donlan said each club paid "about $15.8 million for players" last year (which includes $4 million per club for such collective bargaining agreement guarantees as insurances, pension, severence and injury protection). He said if player costs rise by 20 percent this season, "we'd be paying about $500 million for players next year. There's no way we can give every cent from TV to the players."

Hearing Donlan's mathematics, M.J. Duberstein, NFLPA director of research, said, "Slick, but no cigar." Duberstein said an NFLPA analysis last year showed there is a 25 percent annual turnover on NFL rosters and that usually "in their final two roster cutdowns, teams are shedding higher-priced veterans, who already have negotiated agreements for the next year, and keeping players with smaller contracts. The guys they kept were making about $50,000 per year less than the guys they released."