WESTPORT, Conn. (AP) _ The deadline for a limited partnership including actors Paul Newman and Keir Dullea and writer Robert Ludlum to purchase the Westport Country Playhouse has been extended for three weeks, their lawyer said Wednesday.

The buyers acknowledged they have yet to raise enough money for the $300,000 down payment, but their attorney said this had nothing to do with the delay. The agreement was extended at the request of the seller, said lawyer Jacquie Lazo.

The agreement, first signed in January, was extended until June 25, the day after the summer season at the theater is to begin, said James B. McKenzie, president of the Connecticut Theatre Foundation.

McKenzie also is the theater's executive producer and general partner in the group planning to buy the theater, a century-old barn on 31/2 prime commercial acres downtown, for $1.2 million. The partnership was formed by the foundation, which has leased and operated the theater since 1973, producing off-Broadway-style plays.

Ms. Lazo said the deadline extension came ''at the request of the seller. He had that right under the contract. If he had wanted to close today, we would have been ready, willing and able to do so.''

She would not comment further.

The playhouse's current owner, Phillip Langner, has an unpublished New York City telephone number and could not be reached for comment Wednesday.

The playhouse has scheduled 11 productions for the summer. Since first opening 54 years ago, it has been the site of about 600 plays featuring, among others, Henry Fonda, his daughter Jane, Ethel Barrymore, and Mike Nichols and Elaine May.

McKenzie said Wednesday that, on paper, the group has raised all of the $300,000 down payment, but that only about $200,000 actually has been donated.

''I can't tell exactly,'' he said, ''but if all the money comes in we'll have enough.''

He said some of those who had pledged money during a fund drive earlier this year ''are still clearing things with their lawyers.'' He said the partnership still needs ''about $100,000. We could use a few more investors.''

The foundation had to scramble after Langner, whose parents bought the land in 1930, decided in November 1984 to sell it ''for estate reasons.''

The foundation, which had the right of first refusal, scurried to raise money through pledges after it was announced the property was up for sale.

Asked if there were a chance the partnership would fall short, McKenzie said:''Of course not. We're very close.''

McKenzie said the summer theater season will not be affected. It opens June 24 with a production of ''One for the Tango'' starring Leslie Caron.