President Tito of Yugoslavia has set up a special commission to investigate how much influence his wife Jovanka wielded over political and military appointments, Yugoslav sources revealed today.

The sources said the Jovanka who has not been seen in public for over three months, is being held in the presidential palace in the leafy Belgrade suburb of Dedinje pending the outcome of the investigation. The president who at 85 is more than 30 years older than his wife is understood to be staying at another residence about a mile away.

Unlike wives of senior officials in some other Communist countries for example Romania where President Ceausecu's wife is a key member of the ruling Politburou. Mrs. Tito never held any formal post.

Her influence derived solely from her position by Tito's side for over 25 years - a period during which many of his older comrades died of were disgraced. From being his hostess, she gradually became his nurse, his bodyguard and in more recent years, one of his closet advisers.

Yugoslavs have been aware for sometime that Jovanka, who first met Marshal Tito during the war, was in disfavor but the nature of her disgrace has been a closely guarded secret known only to an inner circle of Communist leaders.

From the tragmented details that are now leaking out from this select group a picture emerges of a woman who used her position to control access to the President. Jovanka does not appear to have been particularly interested in specific policies - Yugoslavia is known for its unorthodox brand of communism - but evidently she did try to influence some senior appointments.

The name most often mentioned in connection with Jovanka is Gen. Djoko Jovanic who held the post of assistant minister of defense until last year. Like Jovanka, Jovanic is a Serb from the Lake district of western Croatia. Their acquaintance goes back to the World War II when Jovanka then a beautiful young guerrilla in Titos' anti-Nazi army, served under Jovanic's command.

According to one account, she lobbied for Jovanic to become minister of defense in place of the present minister. Col. Gen. Nikola Ljubicic. It is reasonable to assume that she would have felt that Jovanic would be more favorably disposed toward her than would. Ljubicic, a rather colorless party bureaucrat, in the event of Tito's death.

Tito, who was ill with a liver ailment at the end of last year, does not appear to have been aware of the extent of his wife's activities until comparatively recently. Matters are believed to have come to a head shortly before he was due to leave on a trip to the Soviet Union. North Korea, and China in the middle of August.

Yugoslav sources say Tito may have begun asking his staff questions about Jovanka's influence following a dispute over whether or not she would accompany him on this trip. She customarily went with him on state visits abroad but, according to diplomatic sources she was not invited to the Soviet Union.

Angered by the revelations and by the fact that he had not been told earlier, Tito evidently decided to diassociate himself from Jovanka. At the same time, a number of senior officials in both the president's private office and the Ministry of Defense were moved to less sensitive posts.

High-level personnel changes in recent months have included the transfer of Tito's chief of staff. Mirko Milutinovic, to the post of ambassador in Pakistan. He chief foreign affairs advisers has been named ambassador to India.

In the army, two out of the four ranking generals have quietly been removed. Apart from Jovanic, the deputy minister of defense in charge of national security, Miloa Sunonija, was named as ambassador to Holland earlier this month.

On returning from China in September, Tito was given spectacular receptions in Belgrade and the Croatian capital Zegreb. Nearly half a million people in each city turned out to greet him in massive displays of affection.

At the time, the demonstration in Belgrade was seen as an attempt to match the huge welcomes accorded to Tito in China and North Korea. The Zagreb demonstration was billed as part of festivities marking the 40th anniversary of Tito's election as leader of the Yugoslav Communist Party.

Some observers, now believe that the scale of the demonstrations may also have been intended as a sign of popular support for Tito at a time of personal difficulty. The fact that they took place in both Serbia and Croatia may have been designed to allay fears that the president was favoring one region at the expense of the other.

One of the most damaging aspects of the Jovanka affair to Tito is the so far unconfirmed rumor that it involved attempts to advance Serb interests at the expense of the Croats. Tito has devoted much of his long career to advancing the proposition that all the nations that make up Yugoslavia are equal and should live together in "brotherhood and unity," rather than in the continual feuds of the past.

Theoretically, of course, since Jovanka held no formal position, there is no constitutional reason why she should be formally removed. By setting up a commission to inquire into the affair however, Tito is evidently demonstrating that no one, not even his wife, is above the law.

Yugoslav officials have refused to comment on Jovanka's prolonged absence of other than to say that she is not ill and is living in the presidential residence.

Tito and Jovanka were married in 1952 on his private island of Brioni. News of the marriage was not made public immediately and the first the world knew of it was when invitations to a government receptions in honor of the then British Prime Minister Sir Anthony Eden went out in their joint names.

The couple first met in 1944 in the central Yugoslav town of Drvar. Jovanka then aged 20, was serving as a nurse while Tito was supreme commander of a guerrilla army, known as the partisans, that he had built up from nothing.

Contemporaries have testified to Jovanka's stunning beauty at the time: her handsome face was framed by a partisan cap and shiny locks of thick black hair.

After the war, she was appointed a housekeeper in the president's Belgrade residence. She looked after the wardrobe and also took care of his children by former marriage. They have no children themselves.