The fledgling United Satellite Communications Inc. today said it has obtained financing to begin broadcasting television programs direct from space to home TV sets by this fall--two or three years earlier than expected.

Prudential Insurance Company of America, already a minor partner in United Satellite, said it would invest $45 million in the venture and become its largest investor.

Prudential's cash could put the direct broadcasting satellite on the air this year and give United Satellite a two- to three-year headstart over Communications Satellite Corp. of Washington, which is preparing a similar service.

The United Satellite consortium includes Prudential, General Instrument Corp., a major manufacturer of cable television equipment, and Francesco Galesi, chairman of a private real estate concern, and several venture capitalists.

The company's plan, which requires a series of regulatory approvals from the Federal Communications Commission, is to begin offering satellite-to-home TV service this fall in a triangular region bordered by Boston, Chicago and Washington. Other satellite companies, including Comsat and Satellite Business Systems, have challenged the FCC's initial approval of the plan.

In addition to the Prudential announcement, the company also said one of its partners, General Instrument, had been awarded a $600 million contract for equipment, including home satellite dishes and a combination receiver-decoder box.

Although Comsat's Satellite Television Corp. was the first company to win approval for direct broadcast satellite (DBS) service, its ambitious $800 million plan involves construction of its own high powered satellite, which is unlikely to be launched for at least two years. Subscribers will pick up three channels of programming on two-foot dishes small enough for roof top installation.

"Our system will cost one tenth to one fifteenth of theirs," said Nathaniel Kwit Jr., United Satellite's president and chief executive. "It's an opportunity for consumers to make a quick judgment."

The USCI system will require subscribers to purchase or lease a three- to four-foot satellite dish and will utilize the Canadian satellite, ANIK, which is to be launched this spring. The service plans offer five channels--two movie channels, two news and sports channels using programs from cable networks, and a fifth with other cable programming.

Responding to queries about the United Satellite announcement, Satellite Television Corp. President Richard S. Bodman issued a statement saying the rival's "interim satellite services will help stimulate consumer interest in Comsat's DBS business.

Noting that Comsat had "pioneered" DBS service in the United States, Bodman said the company is "confident we will be the leader in the emerging direct satellite to home industry."