Talks aimed at selecting a buyer for United Press International were expected to last into the night Monday, as negotiators debated the merits of three bids to rescue the financially strapped news agency from bankruptcy.

A committee of the agency's creditors listened throughout the day to a series of presentations from prospective purchasers and the wire service's union, sources said. UPI management also was scheduled to make a presentation.

William Adler, a UPI spokesman, said that management still was undecided about its preference for a buyer. He said, however, that the situation was moving very fast, and that a final choice could emerge as early as this week.

The three prospective buyers are Mario Vazquez Rana, a Mexico City publisher heavily involved with the international Olympic movement; Joe E. Russo, a Houston developer and financier; and a consortium of investors headed by the Financial News Network, a fast-growing cable firm.

Any final choice of buyer, as well as specific proposals for reorganizing the news agency, must be approved by the court that has been overseeing UPI's bankruptcy for the past six months. However, negotiators say they would like to come up with a choice acceptable to management, the creditors and the Wire Service Guild, the union representing 750 UPI employes.

Adler said the three parties have not yet met as a group but have been consulting "in twos" over the news agency's future. Over the weekend, for instance, UPI management met with the union, which had announced last week that it reached agreement on contract terms with Rana and Russo.

UPI reportedly expressed concern that these terms would hinder future management, for instance by establishing too stringent formulas for placing employes on the future board of directors. And one source said that the agreements the union reached are under review, although an official familiar with the union's position said he expects them to hold together.