3 Ex-Yugoslav Republics Are Accepted Into U.N.

Three former Yugoslav republics were formally elected as full members of the United Nations today in an emotional session punctuated by pleas to save Bosnia and Herzegovina from imminent disintegration.

"A human tragedy of catastrophic proportions threatens to erupt in my country if urgent measures are not taken immediately to stem the reign of terror in Bosnia and Herzegovina," said Foreign Minister Haris Silajdzic in accepting membership for his republic. 'We call on this body to come to our aid in the hour of our greatest need."

Also joining the United Nations were Croatia and Slovenia. Most of the speeches welcoming all three countries cast direct or indirect blame for most of the violence in Bosnia and Herzegovina on Belgrade, which has sponsored a campaign by Serbian guerrillas and the virtually all-Serbian Yugoslav Army to carve up the region along ethnic lines.

The new American representative to the United Nations, Edward J. Perkins, in his first address to the General Assembly, went as far as to question the status of Serbia and Montenegro in the organization, saying that these republics, which make up the new, smaller Yugoslavia, should be required to apply for membership and be held to the same standards as other applicants. A Burden of Proof Seen

"They must prove to the members of the U.N.," he said, "that the so-called Federal Republic of Yugoslavia is a peace-loving state."

Last month, the federation formed by Serbia and Montenegro declared itself to be the successor to the old Yugoslavia at the United Nations, and it has yet to be formally challenged. At the session today, representatives of the federation physically occupied the place of the old Yugoslav delegation.

The admission of the three former Yugoslav republics is not expected to have an immediate stabilizing influence in the region. But diplomats say that now that the nations are recognized under the United Nations Charter, it may be easier to initiate stronger actions to help them.

The Security Council, in response to the dire situation in Bosnia and Herzegovina, adopted a resolution last Friday demanding an end to the fighting -- and in particular to the interference of the Yugoslav Army, as well as of Croatian forces in some other parts of the republic. Aid and Possibly Sanctions

The resolution urged Secretary General Boutros Boutros-Ghali to review the possibility of sending a peacekeeping force and emphasized that the urgent need for relief aid there.

Diplomats say the Council has since felt increased pressure to take more forceful steps, possibly including economic sanctions against Serbia, if the situation does not soon improve. The Secretary General has formed a task force to keep watch on events in Bosnia and Herzegovina, and is scheduled to report to the Council on Tuesday.

With the three admissions, there are now 178 members of the United Nations. At a ceremony honoring the new members today, the flags of Croatia, Slovenia, and Bosnia and Herzegovina were raised among the banners of all the others, forming a colorful outline to the United Nations complex.

The flags went up about noon, prompting loud cheers from supporters who had gathered across the street on First Avenue, between 43d and 45th Streets in Manhattan. Soon, the joyous din turned into choruses of "Lijepa Nasa Domovina" (Our Beautiful Country), the Croatian national anthem.