It was more than three New York Giants linebackers who caught up with Washington Redskins quarterback Joe Theismann on Monday night when he suffered a compound fracture to his right leg. It also was the law of football probability.

Somehow, Theismann's career had slipped through a loophole in the law that sentences quarterbacks to being marked men living on borrowed time. Theismann, the National Football League's oldest starting quarterback, had missed only one start (in 1980) since he became the Redskins' starter eight seasons ago. He was 36, going on forever, it seemed.

After surgery early yesterday morning, doctors announced that Theismann would have to wear a cast for three months, meaning that his season is over and his career in jeopardy. Theismann broke the tibia and fibula bones in his lower right leg. He is expected to remain at Arlington Hospital for at least 10 days.

Dr. Charles Jackson, the Redskins' orthopedic surgeon, said in a prepared statement yesterday that Theismann was "recovering satisfactorily and is in good spirits." He added that Theismann's right leg would remain elevated "until a repeat surgical procedure can be done on Thursday. Barring any complications, his wound will be closed and his leg casted, allowing him to be up and walking with the aid of crutches."

At a news conference at Redskin Park yesterday, Coach Joe Gibbs expressed optimism and said he talked with Theismann over the telephone and that the quarterback said, "You're not rid of me yet."

Jackson said he wanted Theismann to rest for 72 hours after surgery before making any public statements. He said Theismann would not be able to take part in football-related activities for at least six months. Training camp for next season is eight months away, and Jackson was optimistic that Theismann could play, despite reservations held by others.

The play was frightening enough to cause any observer to have reservations. The Redskins had called a flea-flicker play, and after he had taken a pitchback from running back John Riggins, Theismann had no escape. His blocking disappeared, and his fountain of youth went dry. He was hit by linebackers Harry Carson, Lawrence Taylor and Gary Reasons, causing him to fall in a twisted, awkward way.

Yesterday, the Redskins were trying to balance the gloomy report on Theismann with the fact that backup Jay Schroeder had rallied them to a 23-21 victory at RFK Stadium to keep their season from going over the edge. Their record is 6-5, and they trail the Dallas Cowboys and the Giants by one game with five weeks remaining.

The Redskins' game against the Steelers in Pittsburgh on Sunday will be the first that Theismann has missed since his rookie season with the Redskins in 1974, when George Allen was the coach and Sonny Jurgensen and Bill Kilmer were the quarterbacks.

There also is uncertainty over the status of Theismann's contract. Early last season, the Redskins released a statement saying that a new agreement had been reached with the quarterback, who won the league's Most Valuable Player award in 1983. His contract, due to expire after this season, was to be extended for two more years, through the 1987 season and Theismann's 38th birthday.

However, the contract was not signed. Neither Theismann nor team owner Jack Kent Cooke would discuss the details.

Yesterday, the waves of sympathy came in from all directions. A hospital spokesman said several hundred telephone calls had been received from fans by mid-afternoon yesterday, from as far away as California.

Kicker Mark Moseley and his wife, Sharon, visited Theismann early Tuesday morning. Tackle Mark May stopped by, too. Other teammates called. George Starke, the 13-year tackle who retired this season, called a team doctor to see when he could visit.

Quarterbacks -- marked men from the past and present who saw the play on television or in person -- had much the same reaction. Queasiness, mostly.

Said Doug Flutie, a former Heisman Trophy winner, now in the U.S. Football League: "I was at home and downstairs I heard my mother scream, so I knew something had happened. Then I saw the replay. It puts fear in your heart. It makes you hesitate and wonder what the heck you're doing playing football . . . It's a scary thought."

Philadelphia Eagles quarterback Ron Jaworski, whose broken leg last season ended his streak of 116 consecutive games started, said: "I almost got the feeling during that streak that I was invincible. I never thought I'd be the guy to get a serious injury. You take the licks, see it on film, then wonder how you keep getting up. Then I broke a leg last year in St. Louis . . . What happened to Joe was sick. Tell him I wish him the best."

Jurgensen and Kilmer, who were at the Washington Hilton yesterday to announce plans for the Quarterback Club of Washington's 20th anniversary dinner, had similar views concerning Theismann's future.

"You just don't know if he can come back," Kilmer said. "Really, it's a mental thing, and it's up to him. I had a similar injury when I was 23, and I didn't want to quit at 23 . . . It's tough, though. He's 36 and may be 38 before he gets back."

"He's never been injured. He takes excellent care of himself," Jurgensen said of the 6-foot, 198-pound Theismann. "With his strength, there's no reason he can't (make a comeback). It's completely up to him as to how bad he wants to, and knowing his competitiveness, I would think he'd want to."

Now, many will ponder what the Redskins' short-term future will hold without Theismann. Perhaps another question deserves to be asked: How has Theismann managed to last so long -- 12 Redskins seasons, preceded by three Canadian Football League seasons -- without being cut down by injury?

There has been a trend in the NFL in recent years toward highly mobile, scrambling quarterbacks, such as San Francisco's Joe Montana, Denver's John Elway and, of course, Theismann. The reason for this, NFL coaches and scouts say, is that linebackers, such as the Giants' Taylor, are bigger, stronger and faster than ever before.

Veteran quarterbacks with limited mobility have been knocked off one by one for different periods this season.

"Nowadays, you see defensive players . . . and these guys are 245 pounds and quicker and stronger than quarterbacks," Jaworski said. "Most quarterbacks weigh about 195 pounds, and when you have (defensive) players with a running 10- or 12-yard start, you just can't get away from them."