Conservative political action committees scaled the financial peaks of federal election campaigns during a just-ended 21 1/2-month period.

Continuing a flurry of spending into late last week, the top three alone brought their combined spending total since Jan. 1, 1979, to $9 million, the Federal Election Commission says.

During a shorter but more active period, the first 9 1/2 months of 1980, the big three together spent $4 million while five brethen right-wing PACs disbursed an additional $3.9 million, FEC data show.

The big three -- the National Conservative PAC, the Fund for a Conservative Majority, and the Congressional Club, which originated as money-raising unit for Sen. Jesse Helms (R-N.C.) -- drew from still-brimming combined treasuries containing $629,000 as of three weeks before Election Day.

Their consolidated receipts in the 21 1/2-month period ended Oct. 15 were $14.5 million, while the five other PACs took in an additional $4.5 million. $1

About 90 cents of each dollar spent by the conservative PACs goes for so-called "independent expenditures," such as television ads, in behalf of candidates who legally must neither consent to them nor cooperate with the spender. Such expenditures do not count against the legal limit on PAC contributions to candidates. Only the remaining 10 cents goes for traditional campaign financing -- direct contributions to candidates.

Looking at independent expenditures as a whole, the FEC found about 120 political committees, organizations and individuals gave -- merely in September and most of October -- more than $7.5 million, raising the total since 1978 to more than $12 million. The aggregate figure includes at least $2 million in "negative" spending intended to defeat targeted candidates.

The candidates favored by the eight conservative units were often the same as those who helped by two pro-gun PACs, which raised $1.8 million Jan. 1, 1979, to Oct. 15, 1980, and by three anti-abortion PACs that took in $1 million in the same 21 1/2 months. Their combined spending in 1980, also to mid-October, was $912,000.

These are among the principal findings in a Washington Post review of the last reports on direct contributions the 100 largest PACs of all sorts -- as measured by either 1979-1980 gross receipts or outlays -- will file with the FEC before tomorrow's election. PACs making independent expenditures after Oct. 15, however, must report them by Mailgram. Nearly all such spending is done by the nonpartisan, independent "philosophic" PACs.

Together, the 100 PACs had 1979-1980 receipts of $63.3 million. Their spending in the first 9 1/2 months of 1980 totaled $21.6 million in direct contributions and $7.2 million in independent expenditures for candidates. The bulk of the money went for House and Senate candidates, with the balance going for presidential aspirants.

Although the election law does not require PACs, of which there are a grand total of nearly 2,700, to account until after the election for their final 2 1/2 weeks of direct contributions, indications are that they are setting records. The estimated price tag just for House and Senate contests has been put as high as $400 million by Richard P. Conlon, director of a group of House members called the Democratic Study Group.

A case in point is the PAC of the National Association of Realtors, which on Oct. 17 approved direct contributions of $302,000 before Election Day to 130 congressional candidates. This comes atop outlays of $1,118,591 (and against receipts of $2,456,367) since 1978.

For many PACs, the two weeks starting Oct. 1 was a peak spending period.

For example, PACs of the United Automobile Workers gave $298,750; the International Association of Machinists, $248,655; the National Rifle Association, $165,813; the National Association of Automobile Dealers, $153,737, and the Associated Milk Producers Inc., $149,750.

Despite such huge outlays, the 100 PACs reported combined cash on hand of $10.6 million on Oct. 15. Union PACs accounted for $4.1 million, while the unaffiliated PACs, including the conservative units, had $1.9 million to spend in the final preelection days, if they cared to.

The 100 PACs in The Post 21 1/2-month study fell into these categories:

Corporations: 15; receipts, $4.2 million; spending (all direct), $2.1 million. Biggest spender: Dart Industries, $223,000. Next came American Family, the top seller of cancer insurance, with $194,000. Such coverage is "a very poor buy," the Federal Trade Commission said in a report Congress has forbidden the FTC to release, The Washington Star reported Tuesday.

Trade associations: 14; receipts, $7.7 million; direct contributions, $4.7 million; independent expenditures, all by the realtors, $65,000. Biggest spender: the realtors, $1.2 million. Next came the auto dealers, $901,000. Two insurance groups gave $737,000, the American Bankers Association, $430,000.

Unions: 33; receipts, 16.6 million; direct contributions, $9 million; independent expenditures, $37,000. Biggest spender: UAW, $1.3 million. Next was Afl-cio, $737,000. Units of four maritime unions gave $1.4 million, of three education unions $496,000, of two railroad and one airline pilots' union, $834,000 and of three public employes' unions, $385,000.

Professional groups: 13; receipts, $6.6 million; direct contributions, $2.6 million; independent expenditures (all by the American Medical Association and the AMA Texas unit) $95,000. Biggest spender: the AMA, $1.7 million, with nine AMA state units adding $477,000. The American Dental Association spent $584,000, and the trial lawyers' PAC, $366,000.

"Philosophical": 21; receipts, $25.7 million; direct contributions, $2.1 million; independent expenditures, $7 million.After the conservative PACs, the biggest spender, with a relatively small $343,810, was the liberal National Committee for an Effective Congress.

Agricultural: 4; receipts, $2.5 million; spending (all in direct giving), $1.1 million. Biggest spender: Associated Milk Producers, $541,000, followed by Mid-America Dairymen, $257,000, and Dairymen Inc., $171,000.