One three-digit number, inadvertently left out of the Clean Air Act, has precipitated a major court battle among environmentalists, the Carter administration and the nation's major electric utlity and oil companies.

The error, which occurred during frantic midnight drafting of the legislation last August, could save industry millions of dollars in clear-up equipment outlays for huge power plants, oil refineries and other facilities.

If the equipment is not installed, environmentalists say, thousands of tons of health-periling pollutants such as sulfur dioxide, hydrocarbons and nitrous oxides will be spewed into the air all over the country.

The issue - now before the U.S. Court of Appeals here - is seen by environmentalists as "the first big test" of President Carter's commitment not to sacrifice the environment to increased energy production. And, as far as they are concerned, the administration has already failed the test and buckled to industry pressure.

As the utilities see it, however, the administration has recognized reasonable "energy-environment tradeoffs," avoiding what could have become a major energy crisis if the power plant and oil refineries had been delayed.

The conflict arises from an extraordinary set of circumstances.

Last August Congress faced an auto industry warning to relax clean air standards or see production halted.

A delicate exchange was engineered in which standards would be strengthened for power plants and industrial facilities, by requiring them to install best available control technology" to clean up emissions.

House and Senate versions were reconciled in a single all-night conference and, as amendments to the clean air law of 1970, passed Congress the next day as it hurried away for summer recess. Section 165, which deals with the new technology, clearly States that it was to take effect immediately.

But in the next few weeks it was discovered that another part of the act, Section 168, detailing which provisions were to take immediate effect, omits any mention of Section 65.

On Oct. 7, the Environmental Protection Agency issued a memorandum declaring that the best available technology standard would apply to any plant not under construction as of Aug. 7, when the bill passed.

The agency also asked Congress to pass what's known as a "technical" amendment clarifying the inconsistency between Section 165 and 168.

But "all hell broke loose," recalls Robert Rauch, an attorney with the Environmental Defense Fund.

Industry lobbyists rushed to stop the amendment. George C. Freeman, an attorney with the Richmond-based las firm of Hunton and Williams, which represents dozens of utilities, enlisted Energy Department officials who argued against EPA's position in the White House.

Utilities and industries in dozens of states contacted their senators. Among those who wrote EPA protesting the ruling were Sens. John L. McClellan (D-Ark.), Dale Bumpers (D-Fla.), John Tower (R-Tex.), Birch Bayh (D-Ind.) and Lloyd Bentsen (D-tex.).

Although Edmund Muskie (D-Maine) and Robert Stafford (R-Vt.), ranking members of the Senate committee, said that the technology requirement should take immediate effect, it was clear that a major floor fight would be required.

It was then, as one top EPA official put it, that "Congress punted to us. They didn't have the courage of their convictions and wouldn't fix it with a technical amendment."

On Oct. 27, EPA reversed its position, declaring that any facility which received a permit before March 1, 1978, would not have to comply with the new law.

As one Senate staffer saw it, "EPA didn't have the nerve to withstand the utility industry pressure."

At the time, however, EPA apparently did not realize the implications. Assistant Administrator David Hawkins estimated that only three power plants would be affected.

A Sierra Club survey of EPA's regional offices in January, and subsequent court testimony, revealed that the deadline extension from August to March allowed 94 major industrial plants - many of which rushed to file for permits - to avoid the new law.

These included more than 30 coalfired power plants in Texas, Louisiana, Missouri, Montana Iowa, Pennsylvania, Florida, Alabama, New York, Nevada, Wisconsin and other states, most of which would therefore not he required to install "scrubbers" to remove sulfur dioxide.

Also exempted were about 20 major oil facilities, including three New Jersey refineries to be built by Exxon, Mobil and Sea View Petroleum, the Pittston refinery in Eastport, Maine, and the Hampton Roads refinery in Virginia.

Other industries also slipped in under the deadline. Among them: the East Alco aluminum plant in Frederick, Md.; Transco synthetic natural gas plant in Delaware County, Pa.; a Republic Steel coke battery in Warren, Ohio; a lime plant in Chicago.

On Feb. 17, the Environmental Defense Fund, a nonprofit group with 45,000 members, filed sult to force EPA to apply the technology requirement as of August. An injunction stopped EPA from issuing permits for a few weeks but was lifted Tuesday. The merits of the case are expected to be argued in a few weeks.

For the defense fund, the issues go beyond the 94 plants. "There is only so much clean air," says Rauch. "Once it is used up - and an area is filed with the maximum amount of pollution allowed under thw law - then EPA will have to start telling people EPA will have to start telling people they can't build.

"By allowing these industries to dirty the air on a first come, first served basis, you use up an increment of clean air for the next guy who comes along. Growth could be severely restrained - but more likely, the companies will go back to Congress and will go back to Congress and demand that the clean air standards be scrapped."

Already, Rauch said, several of the 94 plants will use up all the cleanair in their areas. He cited clean air in their areas. He cited Colstrip power plants in Montana and a coal plant in Trimble County Wis.

Freeman contends, however, that without the technology requirement many of the plants could burn low-sulphur coal and emit fewer pollutants than if they burned high-sulfur coal with pollution control equipment - an assertion EDF sharply sharply contests.

The administration, Freeman said, should should be congratulated: "They were forced to make trade offs between energy and environment and they made them in a reasonable way."