Thirty-four persons were injured, three of them seriously, when fire swept through the fifth floor of a dormitory at George Washington University early yesterday, forcing 900 shivering student residents into the darkness.

The fire, which city fire officials traced to a burning mattress, trapped dozens of students in smoke-filled rooms, panicked others onto spotlighted windowsills high above the street and sent two of them leaping five floors onto a gravel courtyard spiked with cone-shaped air vents.

One of the students who jumped, Robin Janicker, 17, of Wappinger Falls, N.Y. underwent surgery at Georgetown University Hospital last night for severe back and leg injuries. The second, William Sims of Cleveland, suffered multiple internatl injuries and was under intensive care at George Washington University hospital.

Both were listed in serious condition.

Many of those injured said they heard fire bells when the blaze broke out about 3:45 but simply turned out and went back to sleep.

In Thurston Hall-a nine-story, coed dormitory plagued for years by student pranksters, occasional vandalism and constant noise - they thought it was just another false alarm.

They did not think so for long.

Choking black smoke billowing from flaming mattresses sent students screaming toward the exits past plaster-covered walls one student described as "bubbling with heat."

One freshman ran from his room barefoot onto "a carpet of fire," another student said. Part of his hand peeled off when he grabbed an overheated doorknob to a stairwell escape route.

Most of the injured students suffered from smoke inhalation and were taken to local hospitals, treated and released.

D.C. fire officials said the exact cause of the fire was still under investigation. Some students who lived on the fifth floor of Thurston Hall at 1900 F St. NW said the fire started when a burning cigarette fell onto a mattress while the smoker was out of the room.

When firemen arrived on the scene, students were hanging from windows, perched on ledges, jumping from lower floor windows and screaming for help.

"They weren't laughing and mocking us this time," said battalion chief T.R. Coleman, who said that Engine Co. 23 has responded to as many as three false alarms at Thurston Hall in a night.

It's only seven blocks from Coleman's station house at 23rd and M Streets NW, to the dormitory and "We respond to the calls no matter what we think," Coleman said.

"We bitch on the way back. But I'd tell those kids-sooner or later, the real thing is going to happen, and nobody's going to be laughing then."

One student said false alarms have been occurring in Thurston Hall for the past two years.

"It's like crying wolf," he said.

Coleman said his men encountered "heavy fire" when they reached the fifth floor of the dormitory.

In the room where the blaze started they found two students sheltered in a bath tub flooded with cold water.One of the students was burned severely. Other firemen carried or led scores of students through the smoke-filled hallway and down the stairwells to safety. Still other residents were plucked from window sills by firemen on "cherry picker" extension ladders.

The false alarms in student dormitories have been a constant problem for university officials. Last year, after a dormitory fire killed 10 students at Providence College in Rhode Island, the GW University staff stepped up fire warnings and posted news photos of the blazing dormitory on bulletin boards.

Students complained yesterday that in addition to the false alarms, the manual alarm system was being tested constantly, and they were confused about what was real and what was not.

In March, an 18-year-old student was seriously injured when she fell responding to a false alarm.

Campus security reported 18 fires, most of them small trash can blazes, and 17 false alarms in Thurston Hall since July 1978.

Under a new D.C. law which took effect last June, university dorms are required to have automatic smoke detectors in every sleeping area. However, the law allows a three-year period until June 1981 to have the devices in place.

Fran Marsh, public relations director for the university, said a design for the alarm system has been approved and a contract will be let this summer.

"I'm so lucky I wasn't stoned last night," one student said. "I wouldn't have ever got up."

Steve Tiffen, a freshman, said he was asleep when "all of a sudden my roommates jumped up and started shaking me. They were in a total state of panic.

"My roommate, Bruce Austin, ran up to the door and said 'We got to get out of here." I said wait, feel the door first." Instead of feeling it eh opened it, and flames shot in and he was burned, really bad."

Austen, of Collinsville, Ill., was reported in serious but stable condition last night in the burn unit of the Washington Hospital Center.

Another roommate ran out into the hall, tripped and fell. "I dragged him over to the window and he wasn't breathing. I started hitting him and pounding him on his back and he started to cough and breathe again," Tiffen said.

In front of the dormitory, students huddled on the street in nightgowns and pajamas in the 40-degree night.

Although evacuation went relatively smoothly there were bad moments for many. Pouring down from the ninth floor, herds of students fell onto each other as a thick wall of smoke boiled into the fourth-floor stairwell.

One student said he found himself halted before a long-ignored, graffiti stained wall that read, "There must be some way outta here, said the joker to the thief."

"I panicked," said Mark Crawford. "I knew it was an omen or something to get even for the way we had carried on [in the dormitory]."

With wet towels wrapped around their faces and their hands and clothes black with soot, small parties of students banded together in sometimes valiant efforts to save themselves.

They broke down doors to rescue trapped students and herded them screaming through smoke and flames down stairwells to safety. Many students also credited building security guards who stayed in the dormitory with calming the residents and preventing more from leaping out of windows.

One student recalled seeing Janicker screaming at her fifth-floor window about to leap. "I heard someone shout, 'stay where you are, don't jump.' And she said, 'I can't, there's too much smoke," and she jumped."

Lying crumpled on the ground below with an injured back, Janicker asked fellow students: "My rabbit, my rabbit-is it okay?" she was told her rabbit had been rescued.

In addition to the three seriously injured, 11 students remained hospitalized last night with various injuries. At least 20 others were treated at emergency rooms and released. CAPTION: Picture 1, GW students, some of them dressed in robes and blankets, gathered near 19th and F streets NW early yesterday after being routed from burning dormitory. By Linda Wheeler-The Washington Post; Picture 2, Firefighter at left instructs George Washington student how to use oxygen mask in lobby of Thurston Hall.; Picture 3, The effects of the fire were apparent in room 533, where fire allegedly started.; Picture 4, a fireman helps an injured student out of the smoke-filled dormitory. Photos by Linda Wheeler-The Washington Post; Picture 5, Firemen clean up debris in Room 533, including the remains of a burned bed.