The nation's newspapers reached the bottom of an earnings slump in the third quarter and may be poised for a modest turnaround in the final period of the year, industry analysts believe.

"I think we're passed the bottom," said media analyst J. Kendrick Noble of Paine Webber Group.Although analysts see only modest increases in advertising revenue in the fourth quarter - after poor third-quarter results - they say that the sharp rise in newsprint expenses has begun to slow.

The year-old national advertising slump was especially hard in the third quarter, which is traditionally soft because of the summer lull in spending.

"A softness in advertising volume continues to adversely affect revenues and operating profits at our advertising revenue-based businesses," said Times Mirror chairman Robert Erburu in a statement Thursday containing the company's earnings report.

But analysts said the fourth quarter will benefit from increased advertising in the Christmas season and from an upturn in political advertising ahead of the November presidential election.

The combination of higher newsprint costs and lower advertising conspired to make the third quarter the worst in many years for the newspaper industry, analysts said.

Times Mirror said third-quarter earnings rose to $81 million from $70.6 million a year earlier, but operating profit before the sale of some timberland declined 21.4 percent in the quarter.

The New York Times said lower advertising revenues and higher costs for its expanded national edition resulted in an 18.1 percent decline in third-quarter earnings to $26.7 million.

Miami-based Knight-Ridder Inc., parent of the Miami Herald and Philadelphia Inquirer, reported flat third-quarter earnings of $32 million on Friday. "Revenue growth from newspaper operations was a disappointing 3.2 percent," Knight-Ridder chief financial officer Robert Singleton said in a statement.

Knight-Ridder, however, reaffirmed that 1988 earnings per share will top the $2.65 earned a year ago. After its earnings were released, Knight-Ridder shares on the New York Stock Exchange rose $1 to $45.75.

The New York Times also expects results for the entire year to top the $160.3 million earned in 1987.

Gannett Co. Inc.'s third-quarter earnings rose to $75.1 million from $70.5 million in the year-earlier period, with a 1 percent gain in newspaper advertising linage.