The U.S. Supreme Court refused yesterday, for the second time, to hear an appeal by Louis Pomponio Jr. against his U.S. bribery conviction for which he was sentenced to three years in prison in 1975.

According to Assistant U.S. Attorney Frank W. Dunham Jr., who has prosecuted many of the government's cases against Pomponio, the Arlington-based real estate developer can asks the Supreme Court within 25 days to reconsider today's order or else he must serve the three-year sentence imposed by U.S. District Judge Albert V. Bryan Jr.

Pomponio unsuccessfully had appealed to the Supreme Court in 1975 to reverse this 1974 convict before he was sentenced.The bribery case involved the payment of $300,000 and two new cars to Sidney Zneimar, former executive and vice president of the Royal National Bank in New York, between 1967 and 1972.

Pomponio was convicted of bribery along with his firm's attorney, Charles J. Piluso Jr. Piluso served part of his two-year sentence and has been released on parole.

Pomponio also faces a one-year prison terms and a $10,000 fine that he received last June from Judge Bryan for a mail fraud conviction. That case, in which Pomponio was found guilty of interstate transportation of fraudulently obtained stock certificates, securities and a $2 million cashier's check, is under appeal before the Fourth U.S. Circuit Court of Appeals awaiting oral arguments.

When Judge Bryan imposed the one-year sentence last June he ruled that it should run consecutively to the three-year bribery sentence if that conviction were upheld. The Fourth Circuit of Appeals upheld that conviction last November, and the Supreme Court yesterday let it stand. So Pomponio could face a four-year prison term if the mail fraud conviction is not overtuned.

Two younger brothers of Louis Pomponio, Peter and Paul, were acquitted of the mail fraud charges last June but are awaiting a decision by the Fourth Circuit Court of Appeals on a 1974 tax fraud conviction that was upheld by the U.S. Supreme Court last October.

That case, in which Piluso was also found guilty, has been referred back to the lower appellate court for consideration of other possible errors that might throw out the conviction, Dunham said yesterday.

Should their conviction be upheld, Peter and Paul Pomponio and Piluso each would face up to three years in prison and fines of $5,000.

The Pomponio brothers ran a multimillion dollar real estate empire in northern Virginia until their financial and tax affairs came under investigation by the Internal Revenue Service and the Justice Department in 1972.