A combination of three of the most widely used AIDS drugs has successfully kept levels of HIV at bay for at least two years, a study published on Saturday showed.

But Dr. Roy Gulick of Cornell University Medical College in New York said results of the longest-ever continuing trial of so-called triple combination therapy showed all three drugs had to be given at once to obtain maximum impact."Large numbers of people are failing triple drug combinations . . . we can say one factor is how these drugs are prescribed," Gu-lick told a seminar before the 12th world AIDS conference.

The findings of the study, which has monitored 97 HIV-infected people for 100 weeks, will support the current gold standard treatment of AIDS using two classes of drugs known as reverse transcriptase inhibitors in combination with one protease inhibitor.

The addition of protease inhibitors to the cocktail of AIDS drugs caused major excitement at the last AIDS conference in Canada two years ago and has already saved thousands of lives.

The study found 78 percent of patients given zidovudine, lamividine and indinavir simultaneously saw a marked and sustained fall in levels of HIV in their blood.