Pittsburgh plans to sell Three Rivers Stadium to an unidentified group of New York investors for $125 million to raise money for the proposed sale of the Pirates baseball team, Mayor Richard S. Caliguiri said.

The sale of the 58,429-seat, open-air stadium, which opened in July 1970, would provide the city's share of the estimated $50 million needed to buy and operate the financially ailing National League franchise during the next five years.

A group of private and corporate investors, led by U.S. Steel Corp. and Westinghouse Electric Corp., plans to raise $25 million to buy the team with an additional $25 million loan from the city.

The Pittsburgh investors recently reached a purchase agreement with the Pirates' current owners, the John Galbreath family of Columbus, Ohio, and Warner Communications Inc. of New York. The current owners claim the franchise lost $6 million in 1984 and predicted it would lose $10 million this year.

Caliguiri said the sale of the stadium will keep the Pirates in the city at an annual cost to taxpayers of $852,000, the cost of leasing the stadium back from its new owners. The mayor said he has asked Allegheny County to pay half of the leasing costs . . .

"Late Night" host David Letterman has talked about Kansas City Royals shortstop Buddy Biancalana. Now, he will get a chance to talk to him. Biancalana, whose sparse batting average was the source of comments throughout the regular season, has been signed to appear on Letterman's show Wednesday night (WRC-TV-4, WMAR-TV-2), according to a spokesman for Advantage International, the Washington marketing company Biancalana signed with yesterday.