The top leaders of three religious faiths - Christian, Jewish and Islamic - yesterday formed an unprecedented Interfaith Conference of Metropolitan Washington, designed to focus a unified religious effort on community problems.

Leaders of 12 Protestant denominations (including seven different kinds of Baptist), two Islamic groups, the Roman Catholic Archdiocese of Washington and three Jewish groups gathered at the Washington Cathedral and signed documents to create the new organization.

In addition leaders of three Eastern Orthodox communions signed as observers, with the understanding that they probably would become full members in the future.

A statement of purpose set the Conference's goals as "speaking with a united voice" on common concerns, "working to transform society" and "trying to become a symbol of unity."

Canon Lloyd Casson of the Washington Cathedral, one of the architects of the new organization, put its objectives more simply when he said: "Our primary concern is to improve the quality of life in the Washington area."

Interfaith organizations including Christians and Jews have become relatively common in recent years. But the Interfaith Conference launched yesterday is only the second organization of religious leaders in the nation to include Muslims. Southern California had the first.

Signing the formal incorporation papers yesterday were Imam Khalil Abdel Alim of the World Community of Islam in the West (formerly known as Black Muslims), Daniel Mann of the Jewish Community Council and the Rev. Dr. Ernest Gibson of the Council of Churches. William Cardinal Baum, Roman Catholic archbishop of Washington, who was ill with the flu yesterday, will sign later.

Although the Interfaith Conference is not schedule to go into full operation until next January, yesterday's founding members established five program priorities. They are problems of the aging, criminal justice, human rights, economic justice and world peace and international security.

They also elected Episcopal Bishop John T. Walker of Washington as president; Rabbi Eugene Lipman of the Washington Board of Rabbis, and Imam Alim as the vice president; the Rev. Richard Taylor, executive of the Christian Churches, as treasurer, and the Rev. Leamon W. White of the D.C. Baptist Convention, as secretary.

The ground rules of the organization attempt to enable the diverse group to speak with a united voice on social issues in cases in which they can, but without overriding the convictions of members who hold different views. "We have been careful not to deal with those areas, however important, where we know we will defer theologically," Mann said.

The conference also is intended "to provide a unified moral force and voice in this community in time of crisis," according to its constitution. Had the body been in existence at the time of the Hanafi Muslim takeover two years ago, Casson speculated to a visitor, "a unified voice might have moved into action more quickly . . . If their religious brothers had spoken to the Hanafi, it might have changed the situation," he said.

The nature of the conference's attack on the five priority areas designated yesterday will be determined by a 13-member administrative group, which will bring specific recommendations to the conference's next assembly, Jan. 11. Casson said he doubted that the recommendations would, initially at least, take the form of specific projects such as the actual constructions of housing for the aged.

In his remarks to the group yesterday, Bishop Walker told the religious leaders that the new organization "has the potential for being one of the most effective instruments for interfaith cooperation in the entire nation."