Two large banks reported improved third-quarter earnings yesterday, but at two others -- Manufacturers Hanover Corp. and Chemical New York Corp. -- results were flat.

In another earnings report, Gannet Co. said that its profits rose 19 percent in the third quarter on a 15 percent revenue gain.

Manufacturers Hanover Corp., holding company for the fourth-largest bank, earned $88.6 million ($1.69 a share) in the third quarter, compared with $88 million ($2.20) last year.

MHC said the gain was achieved despite a $10.8 million impact on net earnings from nonaccrual Argentine loans. Total nonaccrual and restructured loans on Sept. 30 amounted to $1.8 billion, with $696 million of the bank's total Argentine exposure in this classification.

"In light of Argentina's recent agreement with the International Monetary Fund, management believes these loans will ultimately be brought current," the firm said. MHC has roughly $1.3 billion in loans to Argentina, the largest of any U.S. bank.

MHC's provision for loan losses was $104.8 million in the quarter, up from $40.3 million a year ago.

Chemical New York Corp., which also is based in New York, said it had third-quarter net income of $77.2 million ($1.42 a share), compared with $76.3 million ($1.50) a year earlier.

Nine-month, net income rose 8.2 percent to $235.3 million ($4.45) from $217.5 million ($4.58).

Chemical said the declines in per-share earnings for the quarter and the nine-month period reflect higher dividends on adjustable-rate preferred stock and additional common shares outstanding.

Chemical said it has enjoyed higher net interest income, significantly stronger service fees and increased profits from foreign exchange activities. However, it said it sustained a rise in noninterest expense and losses on trading account activities.

Chemical's loan-loss provision for the nine-month period was $113.1 million compared with $98.5 million a year ago. Total assets as of Sept. 30 were $55.2 billion and total deposits were $35.8 billion.

J. P. Morgan & Co., holding company of the nation's fifth-largest bank, yesterday reported a 19.1 percent jump in third-quarter net income over a year ago.

Morgan, parent of Morgan Guaranty Trust Co. of New York, said it had net income of $120.4 million ($2.70 a share) for the third quarter. A year earlier, it posted net income of $101.1 million ($2.30).

For the first nine months of 1984, Morgan posted net income of $370.1 million ($8.34). That was an increase of 10.8 percent over a profit of $333.9 million ($7.65) a year ago.

Morgan said its provision for possible credit losses totaled $120 million in the nine-month period, compared with $185 million last year.

As of Sept. 30, it had total assetsof $62.2 billion and deposits of $36.9 billion.

Security Pacific Corp., which is based in Los Angeles, said it had net income of $74.8 million ($2.04 a share) in the third quarter compared with $67.4 million ($1.83) a year ago.

For the first nine months of the year, the parent of Security Pacific National Bank said net income rose 9.1 percent to $211.3 million ($5.74) from $193.7 million ($5.31).

Gannett Co. Inc., which has interests in newspapers, broadcasting and outdoor advertising, reported profits of $54.9 million (69 cents a share), up from $46.1 million (58 cents) in the same period in 1983. Revenue rose to $473.2 million from $412.6 million a year ago.

Nine-month net income was $149.4 million ($1.87), up 14 percent from $130.9 million ($1.64) a year earlier.