Jean Delahanty had just turned on the television set Monday afternoon when "this big picture of my husband" came shockingly into view. Horrified, she watched as her husband, a D.C. police officer, and two other men were felled by a would-be assassin's bullets.

"I didn't even know he was with the president," Mrs. Delahanty said yesterday from the Washington Hospital Center, where her husband, Thomas K. Delahanty, remained in fair condition with a .22-caliber bullet lodged in his neck near the spinal column.

Delahanty 45 and Timothy J. McCarthy, a 31-year-old Secret Service agent, were both wounded in gunfire that struck President Reagan once as he emerged from addressing a union convention at the Washington Hilton. James Brady, the president's press secretary, also was wounded and in critical condition yesterday.

McCarthy, listed in good condition yesterday at George Washington University Hospital, had been assigned to the presidential protection detail for the last two years. Walking in front of Reagan as the presidential party left the hotel, McCarthy "positioned himself between the assailant and the president" when the gunfire started, and took a bullet in the stomach, according to a Secret Service spokesman.

Delahanty, a 17-year veteran of the D.C. force now assigned to the K-9 Corps, was standing guard at the hotel, entrance, helping with crowd control, when gunshots rang out, according to a police colleague. The only reason he was part of the president's security contingent at all was because his police dog. Kirk, was ill.

President Reagan's eldest son and daughter, Mike and Maureen, made an early afternoon visit to Delahanty's bedside yesterday, staying about 15 minutes. They both expressed concern about his condition and told the injured officer, according to a friend, that "after all this is over, they all ought to get together and have a chat about it."

Later, shortly after 5:30 p.m., Vice President George Bush paid a halfhour visit to Delahanty to check on his condition and to give the police officer a report on the president.

"He just looked wonderful," Bush said after his visit with Delahanty, his wife and other relatives and hospital staff. "He has great spirit."

Delahanty was moved out of the hospital's shock-trauma unit and into a private room late yesterday afternoon. Yesterday doctors said they had decided not to try to remove the bullet from his neck for fear of damaging vital nerves.

"He's in pretty good shape, and he's able to have all his motor functions," a K-9 officer who visited Delahanty reported. "His left arm is numb -- there's no feeling whatsoever, but this may only be temporary."

The officer said Delahanty was lucky not to have been hit by a larger caliber bullet. "It would have completely shattered his spinal cord, but as it was the nerves stopped the bullet so that it only came to rest against the column."

The Delahantys, who live in Beltsville, have no children. Mrs. Delahanty spent Monday night and all day yesterday at the hospital except to keep a doctor's appointment for a recurring eye problem. She was kept company during the day by friends and relatives and consoled by the well-wishes of strangers who wired flowers to her husband's precinct from all over the country.

Bush called Delahanty's wife late Monday and also phoned his condolences to McCarthy's wife, Carol, and the agent's parents, who also received a telephone call from Nancy Reagan. The couple and their two small children live in Dumfries, Va.

McCarthy is the only son of a retired Chicago policeman. His parents flew in from Chicago to be with him, only the first of what his sister jokingly referred to as "thousands of relatives" expected to come to Washington to see the family "hero."

"He can't talk yet because he still has tubes in him," said Laurie McCarthy, the eldest of McCarthy's four sisters. "The biggest problem right now is that he's in terrible pain because they don't want to give him pain killers while they are monitoring his progress." Hospital officials said they could not confirm or deny any such information.

In addition to those wounded by gunfire Monday, Alfred Antonucci, a 68-year-old carpenter from Cleveland, was hospitalized at Georgetown University Medical Center because of an irregular heart heat after he reportedly helped police subdue John W. Hinckley Jr., the alleged assailant. Antonucci, attending the Building Trades union convention at the hotel, was standing immediately in front of Hinckley when shots were fired, according to a union spokesman, and turned to wrestle him to the ground.

Antonucci's daughter-in-law, Eileen, said the conventioneer became "very emotional" during four hours of questioning and was hospitalized for high blood pressure. A Secret Service spokesman could not confirm whether the man had been questioned but said the agency certainly planned to talk to him.