If there was any surprise in the first five days of the third flight of the space shuttle Columbia, it was the ease with which astronaut C. Gordon Fullerton operated the mechanical arm in the shuttle's cargo bay.

Looking into the cargo bay through windows at the aft end of Columbia's cabin, Fullerton worked the arm controls on a console below the windows like a classical organist playing Bach. Even though a television camera on the arm's wrist was darkened by a short circuit, Fullerton was able to use a camera at the elbow as his guide, crooking the arm to make it do everything he asked.

By the third time Fullerton grappled an 82-pound instrument called the Plasma Diagnostics Package with the 50-foot arm, he had the procedure down so cold he was able to take the package from anywhere in the cargo bay and put it back in its berth in four minutes. The flight plan gives Fullerton 25 minutes to stow the drum-shaped instrument.

"There'll be a lot of people reconsidering how to use that arm now that they've seen how easy it is to work," Chester M. Lee, space transportation systems utilization director, said today. "Anybody who's booked a satellite to fly on the shuttle may want to use the arm to deploy it into space instead of the alternative methods we have for satellite deployment."

When Fullerton used the arm to maneuver the instrument into the path of an electron stream fired into space by an electron gun in the cargo bay, he succeeded in staying inside the stream 75 percent of the time.

That was no mean feat, since the electrons were following Earth's magnetic field in space, describing the same erratic curves and arcs that a free balloon might make when its air escapes.

It helped that Fullerton had a magnet inside a thermometer case floating right in front of him so he could follow Earth's magnetic lines of force. Still, his 75 percent was almost twice what a high-speed computer scored when it ran the arm to do the same test back on Earth.

Of the eight instruments in the cargo bay, the only one not working up to speed was a device that measures the strength of the sun's ultraviolet light. A motor that drives filters across the instrument's lens had broken, meaning the instrument could read ultraviolet measurements in only one wave length instead of the six it was built to read.

"I can't tell you how delighted we are with the way the instruments have been worked by the crew," said Dr. Werner Neupert of the Goddard Space Flight Center, which put together the 21,000 pounds of instruments in the cargo bay.

"On a scale of 1 to 10 we were close to a 10 by the middle of the day," Neupert declared, "and we still have two days to go to get to 10."