Two major newspapers have canceled their subscriptions to United Press International and a third says it plans to stop taking the news service, which has been rocked in recent years by mounting financial losses and changes in ownership.

Within the past month, the Dallas Morning News and the Denver Post have canceled their subscriptions to UPI in the middle of five-year contracts. Another paper, the San Jose Mercury-News, has told UPI it does not plan to renew its contract when it expires in October. And there were reports that other newspapers are upset with the wire service.

The reasons for the discontent and cancellations--which could cost UPI hundreds of thousands of dollars a year in lost revenues--range from dissatisfaction with the rate structure to concern about the wire service's new ownership.

UPI officials, however, insist that nothing is amiss. They say they expect to be able to settle the problems with the Dallas and Denver papers and with other disgruntled clients.

"We don't think that UPI is in the midst of any kind of major problems with our clients," said Tom Beatty, UPI's executive vice president and general sales manager.

The major point of contention seems to be the wire service's rate structure, which is outlined on rate cards supplied to subscribers.

Long-time UPI clients are grumbling that new subscribers are being signed up at discount rates, just a few years after UPI--under its previous management--promised to end such discounting.

The Dallas Morning News reportedly cancelled its contract with UPI on March 4 because of the rates being given to newspapers in the suburbs of Dallas. Morning News executives could not be reached for comment.

Darrow Tully, publisher of the Arizona Republic and Phoenix Gazette, is not threatening cancellation, but he met with UPI executives two weeks ago to ask for an explanation of the rate structure and says he wants the wire service to charge all its customers on an equal basis.

Beatty, however, says the only discount UPI has been offering is a new group rate for newspaper chains, which he says is not structured any differently from the wire service's other rates, and which requires the papers to pay full rates within a few years.

Will Jarrett, editor of the Denver Post, said that he "pulled all the plugs" on the Post's UPI news tickers on March 1, and has no plans to resume the $250,000-a-year service.

Jarrett shares the concerns about the wire service's rate structure, but says he cancelled the paper's contract with UPI because of a lack of confidence in its new ownership, which has had little journalism experience.

Jarrett's gripe brings to the surface underlying concerns that have circulated in the newspaper business since a group headed by Tennessee fried-chicken entrepreneur and sometime politician John Jay Hooker took over the wire service nine months ago from the previous owner, E.W. Scripps Publishing Co.

"There's a cloud surrounding the ownership," said Jarrett, who contends that he can break his contract with UPI because it was signed when Scripps owned the wire service. "I didn't sign a contract with these gentlemen," he says. "I signed it with another organization."