Members of the three major unions representing Postal Service employes have overwhelmingly ratified new contracts, officials said yesterday, ensuring 40 months of post office labor stability.

Although the cost of a first-class stamp may eventually rise from 22 to 25 cents, a Postal Service spokeswoman said, the proposal was made months before the negotiations, and labor costs make up only a portion of overall rising operating costs.

The new contracts with the Postal Service were ratified by all three major unions and remain in effect through late November 1990. The final ballots were counted yesterday.

The contracts provide job security language and cost-of-living raises.

Retroactive to July, the deal includes an immediate 2 percent raise, plus lump-sum payments of $250 in July 1988 and January 1989, $300 in July 1989 and January 1990 and $200 in July 1990.

Members of the American Postal Workers Union, which represents almost 350,000 workers, overwhelmingly approved the contract by a margin of 105,786 to 26,851, spokesman Danny Frank said.

The deal was approved almost 7 to 1 by members of the National Association of Letter Carriers, representing about 230,000 employes, by a margin of 112,795 to 16,476, a spokeswoman said.

The Mail Handlers Division of the Laborers' International Union of North America, representing about 50,000 employes, approved the contract by a 9-to-1 ratio, spokesman Drew Von Bergen said. The final margin of acceptance was 17,011 to 1,925.

The contract settlements will avoid lengthy, costly binding arbitration for the first time since 1975, postal spokeswoman Jeanne O'Neill noted.

Postal Service employes are banned from striking, but there were illegal strikes in 1970 and 1978.