Three Democratic members of Congress, returning from an unprecedented visit to a controversial Soviet radar facility, yesterday challenged claims by some Reagan administration officials that the facility is designed -- in violation of the 1972 Antiballistic Missile Treaty -- to help defend against a ballistic missile attack.

Reps. Thomas J. Downey (N.Y.), Jim Moody (Wis.), and Bob Carr (Mich.), who returned Sunday from a surprise four-hour visit to the huge facility near the east-central Soviet city of Krasnoyarsk, said at a news conference here that the radar appeared to be designed only to warn of a ballistic missile attack.

An influential House Armed Services Committee aide who accompanied the lawmakers, Anthony Battista, agreed that the "most probable" function of the radar is to warn of U.S. missile attack. Battista, a senior aide who is an electronics expert, is highly regarded by both Republican and Democratic members of the committee.

A warning radar operating at that site would be a violation only of "the letter of the {ABM} treaty . . . not its purpose," the legislators said in a preliminary report to House Speaker Jim Wright (D-Tex.). They added that the radar appears to be at least two years away from operation, and they reported that the Soviets may be willing to meet U.S. concerns before then.

The radar visit, which occurred at the end of a week-long trip to the Soviet Union organized by the Natural Resources Defense Council (NRDC), was "an extraordinary step in confidence-building," Downey said, adding that it set a precedent for on-site inspections to verify compliance with arms-control treaties "that cannot be undone."

The Soviet Union has never before invited American politicians to secret military installations. The Krasnoyarsk radar in recent years has been the single most controversial Soviet military site after the Reagan administration charged it was in violation of the ABM treaty. Until the photographs and videotapes of the installation taken by the House delegation were brought to Washington, U.S. intelligence agencies had to rely entirely on satellite photos to assess what was going on at Krasnoyarsk.

Several U.S. officials said yesterday they were surprised that the delegation was allowed to visit the Soviet radar. State Department spokesman Charles E. Redman said, "The visit strikes us as a small but welcome step toward increased openness in Soviet military affairs." But Redman also characterized the visit as "part of a Soviet effort to generate maximum positive publicity" on the eve of next week's meeting between Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze.

A White House report last March charged that "the Krasnoyarsk radar and other Soviet ABM-related activities give us concerns that the Soviet Union may be preparing a defense of its national territory." The radar, which administration officials had previously characterized as "fundamentally" completed, "constitutes a violation of legal obligations" under the ABM treaty because it is not situated near a Soviet border, as the treaty demands.

The lawmakers said they were told by senior Soviet scientists that approval for the visit came after all 14 members of the Soviet Politburo, the nation's ruling group, were polled by telephone. The lawmakers said they were also told that the radar is "located within a large security area in which standing orders are to shoot intruders without warning," and that none of the Soviet engineers and military officials at the facility had previously encountered Westerners.

Once at the site, the delegation was permitted to take more than 1,000 photographs and several hours of videotape both inside and outside the main 300-foot-tall buildings that transmit and receive the radar signals. Although reluctant to say so publicly, several members of the delegation acknowledged that their notes and film negatives will be of enormous value to U.S. intelligence experts.

Of particular interest, according to several members, are photos supporting the group's conclusion that the radar facility is of "shoddy" construction and incapable of withstanding the buffeting winds and electronic interference that would be generated by nearby nuclear explosions in the event of war.

If such a radar were built in the United States, Downey said, those responsible "would be court-martialed, and we would have a scandal of major proportions."

Battista said that "hardening" against wind and electronic interference would be essential if the Soviet radar was designed to help defend against a ballistic missile attack, as senior Pentagon officials have claimed.

"Based on what we saw, we judge the probability of Krasnoyarsk functioning as a battle-management radar to be extremely low," the lawmakers said in their report. They added, however, that they also found reason to challenge past Soviet claims that the radar is intended primarily for tracking satellites and other objects in space orbit, a function allowed by the ABM treaty.

Battista said that inspection of the radar's associated electrical equipment led him to conclude that the signal frequency of the radar will be too low for optimum tracking of space objects, although such a use could not be ruled out. "Certainly if we were going to build a space-tracker, it would be at a much higher frequency," Battista said.

Several members of the U.S. delegation said they were convinced that the radar was instead designed to provide early warning of a U.S. missile attack, a function that is permitted by the treaty only if the radar is located at the periphery of Soviet territory. According to U.S. estimates, the radar at Krasnoyarsk is located more than 480 miles from the nearest Soviet border.

"Whether the installation is {designed for} early warning or space-track, it clearly is not deployed," the lawmakers said. "Thus, we judge it to be not a violation of the ABM treaty at this time. However, due to its ambiguous nature, we would no longer be able to make that statement if the project were carried through to completion."

Battista said that when he suggested to a senior Soviet official at the site that construction be halted while U.S. and Soviet officials attempt to reach agreement about the radar, he was told "that is doable."

The delegation included a reporter from The New York Times, aides to Sen. Edward M. Kennedy (D-Mass.) and Rep. Les AuCoin (D-Ore.), and NRDC staff members.