Postmaster General Benjamin F. Bailar announced his resignation yesterday, exactly three years from the day he was named to head the troubled Postal Service.

Bailar, 43, who became deputy postmaster in December, 1974, and postmaster general the following February, will become executive vice president of United States Gypsum Co. in Chicago.

Bailar said at news conference that there was no pressure from the administration for his resignation. "I feel comfortable" in the business community, he said in explaining his departure. He added that he also will make more money than his $63,000 annual postal paycheck.

In his resignation statement, Bailar repeated his often stated opposition to a pending House hall, which he said, "would turn the Postal Service back to the way the post office used to be run."

In 1971, the old Post Office Department was transformed into the independent U.S. Postal Service to be run more like a business than a government agency.

The new legislation would, among other things, restore presidential power of direct appointment of the postmanster general.

At his news conference, Bailer said the bill, H.R. 7700, has "some sections clearly aimed at special interests."

During his three years, Bailar sought to cut costs by reducing the Postal Service's immence labor ofrce.

"The Postal Service is today handling more than 5 billion more pieces of mail a year than it did six years ago," he said yesterday, "and it is today so with 74,000 fewer employees."

While the servi ce has managed to stem the flow of red ink on its balance sheet, it has yet to show a profit. The Postal Service recorded a loss of $688 million for fiscal 1977, against a loss of $1.2 billion in 1976 and $939 million in 1975.

Bailar often got involved in controversy for trying to apply business-like efficency to which was bureaucratic morass. Recently he proposed doing away with Saturday mail delivery, which placed him in a confrontation with mail unions, whose contracts come up for renewal alter this year.

Bailar has been critized by some members of Congress because he has tried to close hundreds of small, inefficent post offices - remnants of the Post Services's political past.

One congressional aide said of Bailar: "He's a capable, smart guy but he never really understood this place at all! You can get along with Congress if you try! But he chose to play his own brand of politics."

Before the 1971 reorganization, the postmaster general held cabinet rank, but now he is chosen by the system's nine man board of governors.

Bailer said the board is scheduled to hold its regular monthly meeting during the first week of March at which time it will consider his replacement. He said he experts to stay on the job "for a few weeks , at any rate."