The Third International Conference on AIDS, which opens today at the Washington Hilton, will be the largest scientific gathering to date on the deadly disease, which has exploded into a worldwide epidemic since it first appeared in 1981.

Up to 6,000 AIDS researchers and 750 reporters are expected for the week-long conference, where scientists from all over the world will present their latest findings on the progress of the epidemic, new experimental treatments for the disease, and the intense search for a vaccine to prevent infection with the AIDS virus.

Highlights of the meeting are expected to include an address this morning by Dr. Robert Gallo of the National Cancer Institute, who said in an interview last week that he planned to announce the discovery of a new, related virus that also infects humans; and a report Tuesday evening by French researcher Dr. Daniel Zagury on results of the first trial in humans of an experimental AIDS vaccine.

Because of the growth of international research efforts and public interest in AIDS, this year's conference will dwarf the two previous ones, according to Dr. George Galasso of the National Institutes of Health, chairman of the organizing committee.

Galasso said 2,200 scientists attended the First International Conference on AIDS, which was held in Atlanta in 1985, and 2,800 came to last year's conference in Paris.

He said 4,200 researchers had registered in advance for this year's meeting, and more than 1,000 were expected to register today. Two thousand scientists submitted abstracts of research findings they wished to report, and about 700 had to be rejected because of time and space limitations, he said.

Conferences of this size are not uncommon in science, but rarely does a meeting on a single disease attract so many researchers. "When you think about it, this is only one virus. So people are interested in all aspects of the virus," said Galasso.

Galasso said the meeting would provide a broad, up-to-the-minute look at research on many aspects of AIDS, but added that he did not expect any bombshells. Two other major scientific conferences on the disease have been held in the United States in recent months, but neither attracted the level of media attention that is expected at this week's symposium.

"If people are going to the meeting expecting to hear something absolutely exciting and brand new, I don't really think that's going to happen," he said.

This week's conference has also become a focus for gay-rights activists and others concerned about the scope of research and public education efforts on AIDS, as well as for politicians and celebrities who have newly adopted the disease as a cause.

Last night, President Reagan called for widespread mandatory testing for AIDS at a fund-raising dinner for the American Foundation for AIDS Research, a private organization chaired by actress Elizabeth Taylor. Outside the restaurant, a crowd of AIDS sufferers and others held a candlelight vigil.

At noon today, a group of demonstrators led by Dan Bradley, a former chairman of the Legal Services Corp. who has AIDS, plans to march from the New York Avenue Presbyterian Church to Lafayette Park to protest what they view as the inadequacy of research efforts on treating and preventing AIDS.

In interviews last week, both Gallo and Galasso said the pace of AIDS research has far outstripped the speed of scientific progress in other fields.

"I think the field is moving at a remarkable rate," said Galasso. "I can understand people who are at risk and who have AIDS who aren't happy with the pace."

Gallo, one of the discoverers of the AIDS virus, said AIDS research "is driving virology, immunology, vaccine science and molecular biology." He said there had been recent advances in understanding how the AIDS virus kills the immune-system cells that are its major targets in the body, and in identifying portions of viral proteins that might best be used to make an effective vaccine.

Gallo said that in his address today he would announce the discovery of a new member of the retrovirus family, the group that includes the AIDS virus (now known as human immunodeficiency virus or HIV), as well as two cancer-causing viruses previously discovered by Gallo and two other viruses recently isolated in West Africa by French and American researchers.

He said he would also announce an advance in the study of Kaposi's sarcoma, a form of cancer common in AIDS victims.

Besides Zagury's report Tuesday evening of his test of a vaccine in human volunteers in Africa, a number of scientists will report new findings on test vaccines in animals, and others will discuss the response of the body's immune system to the virus and its components -- topics central to the search for a vaccine.

Other sessions of the conference will include new estimates of the likelihood that infected persons will develop the full-blown disease; new drugs being tested to treat AIDS; the effect of blood tests and education on preventing spread of the infection; efforts to preserve the blood supply from contamination with the virus; transmission of AIDS among heterosexuals; the spread of AIDS in developing countries, and treatment of the disease's many complications.

Staff writer Sandra G. Boodman contributed to this report.