Pakistan has been forced to ground three squadrons of its meager fleet of jet fighters because the Korean War-vintage planes have become too old and too unsafe to fly, reliable sources said today.

The planes are the U.S. made F86 Sabrejets, once the workhorse of the Pakistani Air Force and considered in their day one of the great combat aircraft. Now, however, they are virtual antiques. According to sources here their age has taken its toll, they have become worthless in combat, and they will be scrapped.

Metal fatigue has attacked the wings of the planes to such an extent that some have cracked under the pressure of putting on the brakes, one source said.

The grounding of the three squadrons, totaling about 40 jets, comes to revitalize its outdated military machine to meet what it sees as a threat from about 80,000 Soviet troops just across the border in Afghanistan.

Pakistan is reportedly seeking funds from some of the oil-rich Persian Gulf states, expecially Saudi Arabia, to rebuild its armed forces. According to one unconfirmed report here, the Saudis are considering giving Pakistan $750 million in return for the stationing of a large number of Pakistani Army troops in the desert kingdom to guard the royal family and the Islamic holy places.

Pakistani defense officials would not comment today on either the grounding of the F86s, which is common knowledge in the foreign diplomatic community here, or the reported deal with Saudi Arabia.

One high Pakistani source, however, indicated negotiations are under way between Saudi Arabia and Pakistan.

President Mohammed Zia ul-Haq, an Army general, has made two trips recently to Saudi Arabia.

The Pakistani Air Force has a total of 256 combat aircraft, of which all but 11 are jet fighters. The three squadrons of F86s are kept at the former U.S. Air Force base in Peshawar, near the Afghan frontier.

Besides the F86 Sabrejets, the Pakistani Air Force has about 140 Chinese-made Mig 19s, which are equipped with Sidewinder missiles and are used as interceptors, and about 65 French Mirages, some of which are interceptors but others are either reconnaissance planes or fighter-bombers.

In addition, the Pakistanis have purchased 32 more Mirage 6s from France for $330 million, and should be getting them shortly.

In 1976, Pakistan wanted to buy from the United States 110 A7 light bombers to protect its borders with India, but the Carter administration refused to approve the sale on the ground that it would increase the arms race in southern Asia.

India, however, has just concluded a $1.6 billion arms deal with the Soviet Union. New Delhi has acquired a significant number of new, more sophisticated weapons during the past four years.

For the past two years the United States has been trying to persuade Pakistan's generals to buy the F5E as a replacement for the F86 because it does well as an interceptor against the Soviet-made Mig21s that both the Afgahn, and Indian air forces are equipped with and because it can be used in support of ground troops.

Pakistan would prefer to skip generations of combat aircraft and go to the F15s and F16s. Those planes are out of the question for Pakistan, however, according to sources here, because there are still not enough of them for U.S. and NATO needs. They would not be available for countries like Pakistan until the late 1980s.

Moreover, they cost between $20 million and $30 million each -- far too much for a country with the economic problems that Pakistan has.