Reports of serious crime increased in Montgomery County during the last three months by 6.2 percent over last year's third quarter, county police reported this week. But the year-to-date number of homicides and rapes in the county remained equal to or below last year's nine-month figures.

Police Chief Bernard D. Crooke characterized the quarterly increase, coupled with a year-to-date increase of 5.4 percent, as "not alarming, but . . . not satisfactory, either."

He noted, however, that at the present rate this year's total of 24,000 serious offenses would still be well below the county record of 30,000 offenses, set in 1980.

Homicide, rape, armed and unarmed robbery, aqgravated assault, residential and commercial burglary, larceny and auto theft are categorized as serious crimes in the county.

Compared with the same period last year, the number of homicides from July through September dropped from four to three and rapes increased from 34 to 45. Robberies dropped from 140 to 135, aggravated assaults rose to 252 from 211 and burglaries dropped to 1,401 from 1,470.

There were nine homicides in the first nine months of this year and last, but rapes dropped from 103 to 95 in the same period.

Other crimes, such as child abuse, shoplifting, drug offenses and vandalism, rose 5.6 percent during the third quarter, and 6.7 percent for the first nine months of this year.

Crooke said he cannot attribute the county's slight but steady crime increase in the last 21 months to any one cause. "So many social factors impact on this issue," he said. Or "it could just be the activity of two or three burglars. They can severely impact the crime rate."

But he said court-ordered early releases of prisoners from jails and a large number of juvenile offenders further complicate the crime problem.

Crooke said too many juveniles, who account for 30 percent of county arrests, are sent to jails that do not adequately rehabilitate them and might actually negatively influence them.

"Some juvenile judges feel like they don't have enough alternative programs to jail or home," he said.

A two-year increase in auto thefts, which rose 18.9 percent in the first nine months of 1985, has also caused concern, Crooke said. His department is studying whether the increase is due more to professional thieves or joyriding juveniles.