Cynthia Gregory, temperamental ballerina with the American Ballet Theatre, has left the company for the third time in five years, according to ABT officials.

Charles France, the company's public relations director, said yesterday ABT was "unable to reach agreement" on a contract with the ballet star. Another ABT official said Gregory "has informed management that she does not intend to perform." Gregory was unavailable for comment last night.

Gregory's apparent resignation came 2 1/2 weeks after ABT locked out its dancers after failing to reach agreement on a new contract. The company is scheduled to begin a four-week run at the Kennedy Center on Dec. 4.

Gregory's name was conspicuously absent from an advertisement ABT ran in Sunday's Washington Post on its Kennedy Center engagement.

Meanwhile, the director of the Stuttgart Ballet, Marcia Haydee, confirmed by telephone that she is trying to hire ABT's newest star, BOLSHOI Ballet defector Alexander Godunov, as a guest star. Haydee said she was informed by Godunov's New York manager that he will be available if the ABT lockout is not quickly resolved.

Godunov, according to ballet industry sources, was hired by ABT primarily to meet Gregory's continual demands for a tall, star-quality partner. In fact, her second "resignation," in November 1977, came because she felt the company had no male partner suitable to her stature and artistic needs.

Meanwhile, a federal mediator said last night she has called officials of ABT and representatives of its locked-out dancers to meet at 2 p.m. today. The meeting will be the first negotiating session since Oct. 31.

Lucia Chase, the co-founder and co-artistic director of ABT, last night expressed optimism that the two sides would reach agreement today to end the lockout. Chase, interviewed outside a Manhattan theatre, literally jumped up and down with glee at the prospect of a settlement and said, "My God, we hope it'll work."

She said she has not been allowed any direct role in the negotiations becuase ABT "won't let me." She would not elaborate on who at ABT she was referring to.

Chase stated that ABT "will make a new offer" today. But an extremely reliable ABT official said last night that the company did not anticipate revising its position unless the dancers did likewise.

At the Oct. 31 session, the federal mediator, Carol Holter, said that both sides were too far apart at that time for additional negotiations to prove fruitful.

Nora kaye, a member of ABT's board and one of the company's most famous dancers during its early years, appeared at a party for the dancers Monday night to express her support for the dancers in their dispute with management.

Kaye, who currently holds the title of co-associate artistic director at ABT, said last night that she decided to support the dancers in full knowledge that she may be asked to resign from the board as a result. If her resignation is requested, she will oblige, she said. She added that she felt the dancers are grossly underpaid and deserve a more substantial wage increase than management has offered.

As of last night, ABT's offer stood at a 22 percent package increase over three years, and the dancers' request was estimated by management to be 75 percent over a two-year period.

Kaye confirmed last night that she had been offered, and had turned down, the directorship of ABT prior to Mikhail Baryshnikov, who will assume the position next September. "I have a husband," she said in explanation. Kaye is married to film director Herbert Ross of "The turning Point," "California Suite," "Funny Lady," and the upcoming film, "Nijinsky," which is expected to be released next February.