The five firms that have sold the more than 2.3 million all-terrain vehicles (ATVs) in use in this country agreed yesterday to stop selling the three-wheeled models that have been described by state officials and safety experts as "killer machines."

The action, hailed by federal officials as a major step toward ending the high rate of injuries associated with ATVs, was attacked by congressional opponents and consumer groups as a pyrrhic victory because the firms had already planned to end manufacture of the vehicles next year.

Consumer groups had pressed for the companies to issue refunds to owners of the three-wheeled vehicles, which account for two-thirds of the ATVs in use. But Justice Department and Consumer Product Safety Commission officials strongly defended the court-approved settlement yesterday, saying it was doubtful they could have forced the manufacturers to undertake the massive refund program that the commission had once sought.

"This is a giant step toward reducing the deaths and injuries that have been associated with these machines," said Deputy Attorney General Arnold I. Burns as he outlined other actions that the firms must take under a preliminary consent decree.

"This isn't so much a settlement as a sellout," countered Rep. James J. Florio (D-N.J.), chairman of a House subcommittee with oversight on consumer issues.

Rep. Doug Barnard (D-Ga.), chairman of another consumer affairs subcommittee, called the agreement "woefully deficient . . . less than half a loaf" and promised to investigate how the settlement was reached.

"This is a pathetically weak action," said R. David Prittle, a former member of the CPSC and now techical director of Consumers Union, publishers of Consumer Reports. Prittle said he was especically troubled that the agreement does little for ATV owners whose machines "are like ticking time bombs."

The agreement does not affect sales of the four-wheeled models that also have been declared an "imminent hazard" by the CPSC and Prittle said are almost as unsafe as the three-wheeled models.

Four of the five firms involved in the settlement, Honda Motor Co., Yamaha Motor Co., Suzuki Motors Co., and Kawasaki Heavy Industries Ltd., are based in Japan. The fifth, which has a relatively small share of the U.S. market, is Polaris Industries, L.P., of Minneapolis.

The three-wheel ATV essentially is a motorized tricycle with a high center of gravity. According to the Justice Department, the problem with the three-wheeled models is that while they appear to be safe with their large wheels and high-powered engines, they have a tendency to flip over when driven by youngsters and others who attempt to do "wheelies" and take the ATVs over rough terrain.

ATVs began arriving in the United States in large numbers from Japan in the early 1980s with a promise that they could "go anywhere," the Justice Department said yesterday in court papers.

But "far from being safe, easy-to-ride vehicles for harmless play," the ATVs have been associated with an extremely high accident rate, involving more than 900 deaths since 1982, the department said. Many of the injured have been children who were thrown off the high-powered machines, which allow virtually "no margin for error" when driven, the Justice Department said.

The CPSC branded the vehicles a hazard in 1986 after the commission came under intense congressional pressure to investigate. The decision to settle the case through a consent decree remained controversial on the three-member CPSC yesterday.

Commissioner Anne Graham said she voted against the agreement during an executive session Tuesday because she was troubled by the industry's "lack of concern or commitment to the consumer's safety."

Sen. Alfonse M. D'Amato (R-N.Y.) described the settlement as "a Christmas present for the Japanese-based ATV manufacturers and a disaster for the American consumer." He said he would introduce legislation to force refunds for purchasers of the three-wheeled models and others sold for use by children under age 15.

Missouri Attorney General William Webster Jr., one of the state attorneys general who have described the ATVs as "killer machines," called the settlement "a very important step," but he stopped short of endorsing it.

The firms that agreed to the settlement did so without admitting that the ATVs were unsafe, or as a Justice Department official put it, "dynamically unstable and complicated to operate."

Under terms of the preliminary decree, signed yesterday morning by U.S. District Court Judge John H. Pratt, a driver-training course must be given to anyone who purchased an ATV within the past 12 months. As part of the agreement, the industry is obligated to mail warning letters to all past ATV purchasers that it can locate and to begin advertising and other steps to educate the public about the hazards of operating the vehicles.

The promise of immediate warnings and other steps the industry will undertake immediately outweighs the cost of a prolonged court battle "with an uncertain outcome," said CPSC Chairman Terrence M. Scanlon.

Asked what a recent purchaser of a three-wheel ATV could do, Burns replied: "The first thing I would hope you would do is to go see a good lawyer."

Prittle and other critics said they were troubled that some of the warnings purchasers must now sign when they buy one of the vehicles will weaken the chances of any owner successfully suing one of the ATV manufacturers for any injuries a rider might receive.

Burns disagreed during an hour-long news conference at which he and CPSC officials found themselves on the defensive over the agreement. The Justice Department did not wait until after the Christmas sales period to sign the agreement as a concession to the industry, said Justice Department spokesman Patrick S. Korten, calling a CBS News report to that effect "utterly and outrageously false."

"We weren't ready to shoot our guns" until yesterday, Burns said.

Justice Department officials said the agreement allows the CPSC to return to court if it finds evidence that the ATV industry is not living up to terms of the agreement or it finds new evidence of increased safety problems with the vehicles. The officials said they expect to enter a final decree in the case in about 45 days.

These are the key steps that makers of all-terrain vehicles (ATVs) agreed to take to settle a safety dispute with the Consumer Product Safety Commission: An immediate end to sales of three-wheeled models.Repurchase of any three-wheel models from dealer inventories.Warning letters to all ATV purchasers.Warning posters in all dealer showrooms.Warning notices in owner manuals.Driver training classes to be offered to purchasers within the past

12 months.A toll-free hotline to be established for ATV owners.Minimum standards to be set for ATV components.Advertising campaign to alert ATV owners about safety hazards.Banning sales or use of high-powered ATVs by children under 12.Place safety warning labels on all ATVs.Will not oppose state proposals to require licenses for ATV

operators.