If the United States became involved in a major ground war tomorrow, it would only be able to care immediately for three out of 10 wounded military personnel, the Pentagon's top health official told a congressional panel yesterday.

The military medical corps, established to answer to the needs of the nation in times of conflict, has been strained by an increasing demand for peacetime health services, said Dr. William Mayer, assistant secretary of defense for health affairs.

And when emergency combat care is needed -- such as it was two years ago when Marines were bombed in Beirut -- the Defense Department isn't adequately staffed to coordinate medical services of the Armed Forces branches, he said.

"We are not ready for war . . . and in my judgment, our priorities have not been correct," Mayer told the investigations subcommittee of the House Armed Services Committee.

Two days earlier, he and Secretary of Defense Caspar Weinberger had made similar statements to senior medical officers during an unprecedented health conference on the quality of military health care.

In hiring for military hospitals, for example, "I do not believe we can afford such luxuries as fully staffing military obstetrics units for 10 deliveries a month or maintaining expensive training programs for medical specialties that are of little or no use in war," Weinberger said.

Yesterday, Mayer described the medical needs of the 10 million active-duty personnel, retirees and dependents who can use the military health care system as "an enormous burden" for that system.

While serving those needs, the Defense Department has not been able to maintain the "proper mix" of doctors that would be needed during times of combat, he said. Defense officials have indicated, for instance, that the system would need more orthopedic surgeons than are on staff now.

Subcommittee members asked Mayer repeatedly about a classified document report dated April 1984 that, based on a review of the 1983 bombing of a Marine Corps barracks in Lebanon, charged that the U.S. European Command lacked the proper resources and plans for emergency or combat medical care.

Memorandums filed by Air Force and Army doctors who cared for Marines hurt in the bombing revealed that interservice rivalry played a disturbing role in deciding where the injured would be treated, members of the subcommittee noted.

The memos show that Air Force and Army medical personnel were jockeying to care for the casualties; the Pentagon commission charged that, as a result, wounded men were taken to an Air Force hospital rather than a better-prepared Army facility.

"In retrospect, it appeared to many that the Army hospital should have been used," Mayer said. "Nothing that the services did . . . in any way worked to the detriment of those patients. But the potential was there."

Mayer, however, could not reassure the committee that similar problems would not recur during combat. Neither the European or Pacific Command has a full-time officer "with clear authority to review and oversee medical planning for all three services," he said, adding that he had requested the establishment of such a position one year ago.

"I would like to say with total confidence that the kind of confusion we've experienced in the past won't happen again. I cannot," Mayer said.