Two astronauts stood ready today to take the space shuttle Columbia on its third flight into earth orbit in less than a year.

This evening, Columbia commander Jack R. Lousma, 46, and copilot C. Gordon Fullerton, 45, had flown their last acrobatics in T38 jet trainers to acclimate themselves to space flight, and shuttle managers were describing the countdown for the 10 a.m. Monday takeoff as the smoothest yet.

"We've seen no problems at all that are a constraint to launch," shuttle test director Norman Carlson said. "This has been a clockwork count, the smoothest any of us has seen since we started this test flight program."

The weather was perfect everywhere: at the launch site here at Kennedy Space Center, at the landing site at White Sands, N.M., and at the alternate landing sites at California's Edwards Air Force Base and Spain's Rota Naval Air Station.

"We expect a little ground fog early in the morning at the launch site that should burn off just after sunrise," Capt. Donald Green, the weather officer, said. "Other than that, clear skies are forecast everywhere around the world we might have to come down and land."

In the next to last of the four flights scheduled to test the spaceworthiness of the shuttle, Lousma and Fullerton will stay in space a little more than seven days and circle the earth 116 times, more than the first two flights combined.

While in orbit, the two astronauts will maneuver the 100-ton shuttle so that the nose, tail and cargo bay will be exposed to the unprotected glare of the sun for long periods to see how it works when it's heated to temperatures of almost 250 degrees. They also will test the engines and fuel tanks to see if they can restart the engines in space after long exposures to temperatures as low as 215 degrees below zero.

At the end of their mission, Lousma and Fullerton will land Columbia at White Sands Missile Range in the southern New Mexico desert. Long an emergency shuttle landing site, White Sands became the prime site when heavy spring rains turned the Mojave Desert beds at Edwards Air Force Base into a quagmire.

Lousma and Fullerton are scheduled to land Columbia at 1:27 p.m. EST on Monday, March 29, which is not the best time of day this time of year in the southwest. March winds pick up in the early afternoon and the desert sun could be a distraction. Lousma and Fullerton will try to land in a crosswind to see how the shuttle handles the aerodynamic forces put on it by shifting winds.

But the crosswinds have been clocked in the last few days as high as 35 knots at both of the White Sands runways in the early afternoon, too high for the shuttle to handle at this point in its test flight program. If it appears that the crosswind may be too high, Lousma and Fullerton may be ordered to land an hour or two early.

Shuttle managers plan to begin landing the spacecraft here on the Kennedy center's concrete runway on the fifth shuttle flight later this year.

They are reluctant to try it until they've finished the program of four test flights and landings on the long, wide desert runways in the western United States. The runway at Kennedy is three miles long and less than 100 yards wide, presumably more than adequate once the shuttle's landing characteristics and aerodynamics are fully understood.

On its first flight last April, Columbia landed at Edwards too far down the desert runway. Its second landing last November came up a little short on the same runway, which is thousands of yards wide and more than seven miles long. The desert runways give the astronauts and their new automatic landing system plenty of margin for error during the early stages of the delta-winged spacecraft's test flight program.

The landing at White Sands will be expensive for NASA. A 23-car train carrying the heavy equipment needed to service the shuttle after landing arrived from Edwards at El Paso, Tex., Saturday night, then was due to travel another 80 miles to New Mexico's Holloman Air Force Base where technicians were to unload the equipment and drive it on flatbed trucks the last 23 miles to White Sands.

Among other things, the convoy contains a mobile wind machine used to blow away any explosive gases that leak out of the shuttle fuel tanks after landing and two giant mobile air conditioners to blow cool air into the shuttle's fuselage, cargo bay, wings and tail to dissipate the heat that has built up on the spacecraft from the heat of reentry into the atmosphere.

The space agency has not said what it cost to move all this equipment the 1,000 miles from Edwards to White Sands, but estimates range from $250,000 to $500,000.