United Press International yesterday filed suit against three former senior editors who resigned in November, charging them with breach of contract and accusing them of defaming the wire service and depriving UPI of more than $505,000.

According to the suit filed in U.S. District Court here, the former editors -- Barry Sussman, Kim Willenson and Ben Cason -- each have drawn $10,500 a month since quitting, getting the money lines of credit set up for them while they worked at UPI. UPI said the resignations were a breach of their five-year employment contracts and thus disqualified them from receiving the funds.

The suit also charged that the statement made by the three when they quit -- that they could "no longer assure the quality and integrity of the UPI report" -- was made "with actual malice and with the intention of damaging UPI's reputation. ... " UPI said clients canceled the service as a result of the statement, costing UPI at least $505,590.

Robert Reznick, the attorney for the three men, said, "The charges are ludicrous. It is incomprehensible that {Mario} Vazquez Rana {UPI's owner} would choose, by filing a lawsuit, to expose his management of UPI to public scrutiny. The three editors acted at all times to the highest personal and professional standards." Asked about a possible countersuit, Reznick said, "We had told UPI that we would not be the first ones to bring the dispute to court. With the filing of the suit, we are considering all available alternatives."

According to the lawsuit, UPI obtained a $316,000 letter of credit for Willenson and a $345,000 letter of credit each for Cason and Sussman from American Security Bank. The latter two joined UPI in January 1987. The letters of credit were intended to provide security for the editors. In the event that UPI, which reportedly is losing at least $1 million a month, failed to pay them, the editors could withdraw $10,500 each per month on the letters of credit, which expire January 1992.

The lawsuit charged that the editors "have unjustly enriched themselves by drawing upon their letters of credit" while not working at UPI. Among other things, the suit seeks compensatory damages for breach of contract, restitution equal to the amount drawn on the letters of credit, an injunction to prevent the editors from drawing more money and to prevent them from working for a UPI competitor until 1992, and damages of at least $505,590 for money lost from clients.