For the third consecutive year, crime in Prince George's County has continued its downward spiral, dropping 6 percent through September, the county police department reported.

The most dramatic decline was in murders, which fell by a third from the first nine months of last year, when 39 were reported, compared with 26 this year. Meanwhile, the number of rapes jumped 18 percent, with 261 reported this year, compared with 221 through September last year.

Maj. James Ross, who heads the county's criminal investigation division, said he had no clear explanation for the drop in murders. Historically, he said, they don't follow other crime trends.

"The statistics of other crimes, like robbery and theft, parallel the economy somewhat," Ross said, noting that unemployment in the region has fallen since last year. "But because most murders occur in the heat of passion, it's difficult to cite any one reason," he added.

The county saw a large decline in robbery, with 845 armed robberies recorded through September, compared with 1,055 recorded for the same period last year, a 20 percent decrease. At the same time, strong-arm robbery for the first nine months was down from 748 last year to 645 through September, a 13.8 percent decrease.

Larceny and theft also fell, by 10 percent, with 12,936 reported through September, compared with 11,648 last year.

Ross also cited increased police neighborhood patrols, school education programs and cooperation from the business community in bringing crime levels down.

The reported 18 percent increase in rape, Ross said, was a result of more women reporting the crime. "This is because they're seen as victims, whereas 20 years ago, they were treated like defendants," Ross said.

He also attributed increased reporting of rape to greater sophistication in methods used to help victims. "When a rape is reported, the police officer who is dispatched contacts investigators, who put the woman in touch with the crisis center," Ross said. "There are simply more channels for women to get help."

The broad drop in crime, which reflects a national trend, is similar to that reported for Anne Arundel County, which reported a 5.7 percent decrease for the same period. In this area, two counties reported increases: Montgomery County, up 5 percent, and fast-growing Charles County, up 12 percent.

In 1981, crime in Prince George's increased 3.7 percent. But in 1982, crime fell 11 percent, and continued that decline in 1983, when crime was off 12 percent, said Robert Law, a police department spokesman.

The department this year also reported an increase in aggravated assaults of 7 percent, from 1,535 for the nine months ended in September 1983 to 1,645 this year. "That's another category that we think is reported with more frequency because their cases are more professionally processed," Ross said.

Meanwhile, residential burglary dropped 4.8 percent, from 4,314 to 4,107, and property crime dropped 6 percent, from 21,883 to 20,527.