A Middleburg horse trainer acquitted earlier this month of trying to kill his estranged wife, whom he found in bed with one of his best friends, has been indicted by a Loudoun County grand jury on a charge of murdering his wife's lover.

Yesterday, a day after he was charged with the murder of horse dealer Howard LaBove, 30-year-old Theodore Gregory surrendered to authorities and was released on a $100,000 property bond. He is scheduled to return to court Thursday for a hearing to set a date for the trial, his third in connection with the lover's triangle slaying that shocked Virginia's horse country.

On Friday, Loudoun County Circuit Judge Carlton Penn granted Gregory and his wife, Monique Dana, a divorce, an action Gregory's lawyers had contested because it frees Dana to testify against her ex-husband about the August 1980 shooting of her lover. Under Virginia law spouses cannot testify about crimes their spouses allegedly committed against third parties.

Gregory's lawyer, Jeffrey Rice, called Penn's decree "somewhat unusual" in that each spouse's divorce was granted on different grounds. Dana's was granted because the couple had lived apart for a year while Gregory's was based on the grounds that Dana committed adultery.

Rice said Gregory will appeal the divorce because it was granted in Loudoun, not in adjacent Fauquier County, where the couple had last lived together. He also said Gregory, who spent more than 18 months in the Loudoun County jail between the time of his first and second trial, had been improperly served with divorce papers in jail.

"I feel wonderful about the divorce," Dana, a 28-year-old riding instructor who has assumed her maiden name, said yesterday. "I'm not looking forward to testifying in another trial, but it has to be done."

Gregory, who has been living on his parents' Fauquier County farm since his acquittal, could not be reached for comment.

Three weeks ago, a jury in rural Westmoreland County, 75 miles south of Washington, acquitted Gregory of trying to murder his estranged wife after he found her making love with LaBove, who was then his business partner.

That trial, which had been moved from Loudoun at the request of the defense, marked the second time Gregory had been tried for attempted murder. In December 1980 Gregory was convicted by a jury. That verdict was later voided after Judge Penn agreed with defense attorneys that he had improperly prevented a psychiatrist from testifying.

Defense attorneys claimed Gregory was temporarily insane and therefore innocent. According to testimony, Gregory followed LaBove--well-known in Middleburg as a womanizer--and Dana from a dinner party, took a .45-caliber pistol from the glove compartment of his pickup truck and peered in the window of LaBove's cottage. He then entered the house, opened the bedroom door and announced, "I'm going to kill you, b----!"

Dana fled naked to a nearby house and plunged both arms through a glass door seeking refuge. LaBove was found dead with three bullet wounds in the chest.