Two of the nation's largest bank holding companies said yesterday that their third-quarter earnings were up 8 percent from the same period last year, and another major bank organization said its profits fell nearly 12 percent.

Citicorp, parent company of Citibank, the nation's second-largest commercial bank, said its earnings for the July-September period rose to $151 million from $139 million in the same period a year ago. Per-share earnings were up a dime to $1.21.

Crocker National Corp., parent company of Crocker National Bank in San Francisco, reported a jump in earnings to $23.3 million from $21.5 million for the comparable 1979 period.Crocker National is the country's 13th-largest bank.

Wells Fargo & Co., whose main subsidiary Wells Fargo Bank in San Francisco is ranked 11th among U.S. banks, said its third-quarter profits fell to $30.4 million from $34.4 million a year ago.

Among other major companies reporting earnings yesterday were Union Oil of California, Monsanto, Lockheed and Republic Steel.

Citicorp attributed its gains to a 16 percent increase in net interest revenue -- the difference between the interest it earned on loans and that which it paid out to depositors. Its foreign-exchange-trading income was $31 million compared with a loss of $4 million in last year's third quarter.

For the first nine months of the year, Citicorp's net income climbed 5 percent to $411 million from $390 million in the same period last year.

Crocker, which is pursuing a proposed takeover by Midland Bank Ltd. of London, said its earnings gains were due to higher net interest income and a smaller provision for anticipated loan losses. For the first nine months of the year, Crocker's net income was up 9.5 percent to $69.3 million from $63.2 million in the same period last year.

Union Oil Co. of California, the nation's 14th largest refiner, reported yesterday that its third-quarter profits rose 15 percent, primarily on higher oil and gas production earnings.

In the latest quarter, Union Oil earned $122 million (71 cents a share) versus $106 million (61 cents) a year earlier. Revenues rose 23 percent to $2.63 billion from $2.14 billion.

Union's nine-month net income was up 35 percent to $469.8 million (2.71) compared with $348.3 million ($2) in the corresponding 1979 period. Revenues climbed 34 percent to $7.76 billion from $5.77 billion.

Fred Hartley, Union Oil's chairman, said improved chemical profits and higher investment-tax credits also contributed to the company's third-quarter and nine-month results.

Monsanto Co., the chemical giant, said yesterday that the third quarter was one of the most difficult in its history, with earnings dropping to $29.6 million (81 cents a share) from $98.9 million ($2.73) a year earlier in spite of a rise in sales to $1.539 billion from $1.497 billion. y

Nine-month profits were $217 million ($5.98) on sales of $4.91 billion compared with $320.2 million ($8.81) a year earlier on sales of $4.647 billion. c

John W. Hanley, chairman and chief executive officers of the producer of chemicals and synthetic fibers, said that three operating companies -- plastics and resins, textiles, and chemical intermediates -- all lost money in the quarter.

Lockheed Corp. said yesterday it earned $16.4 million ($1.03 a share) in the third quarter in contrast with a deficiency a year ago caused by a write-off of excess production costs of the TriStar program.

The $1.1 million in profits a year ago failed to cover preferred stock and securities redemption requirements, and sales were $1.02 billion.

The TriStar program losses in the quarter were $29.3 million against a loss of $69.1 million a year earlier.

Nine-month earnings of $1.8 million amounted to a deficiency of about 19 cents a common share after preferred dividend and securities redemption requirements. The 1979 nine-month earnings were $22.8 million ($1.30). Sales were $3.85 billion against $3.01 billion a year ago.

General Public Utilities Corp., citing the removal of its Three Mile Island nuclear unit one from its rate base, said yesterday that earnings in the third quarter were down 58.9 percent from the same period a year ago.

Earnings in the third quarter were $10.5 million (17 cents a share) on revenues of $490.2 million compared with earnings of $25.6 million (42 cents) on revenues of $383.9 million in the same period a year ago.

Nine-month earnings were $19.2 million (31 cents) on revenues of $1.4 billion compared with earnings of $81.3 million ($1.33) on revenues of $1.1 billion in the same period last year.

Republic Steel Corp., which had braced shareholders for bad news last month, said yesterday it lost $14.5 million in the 1980 third quarter because of the recession and a depressed steel market.

The nation's fifth-largest steelmaker said third-quarter sales of $833.4 million were off 18.9 percent those of the 1979 third quarter, with shipments down 26.5 percent to 1.4 million net tons, the lowest point in recent years.

The loss compared with earnings of $29.9 million ($1.85 a share) in the same 1979 period. Sales for the latest quarter were $1.03 billion on shipments of $1.9 million tons.

"It appears that the third quarter results will constitute the low point in terms of the effect of the recession on Republic." William J. DeLancey, chairman and chief executive officer, said.

"The company is currently experiencing an improvement in demand with the result that industry shipments for this year should be several million tons higher than the 1975 recession low of 80 million," De Lancey added.

Nine-month net income was $15 million (80 cents a share) compared with $115 million ($7.11) a year earlier. Sales were $2.7 billion compared with $3.1 billion in 1979.

Kellogg Co., a maker of breakfast cereals, earned $52.71 million (69 cents a share) in the third quarter, up from $44.69 million (58 cents) a year ago. Sales rose 24.9 percent to $560.72 million from $449.05 million.

Nine-month net income was $150.72 million ($1.97) on sales of $1.613 billion compared with $133.91 million ($1.75) a year ago on sales of $1.38 billion.

Knight-Ridder Newspapers Inc. said third-quarter earnings declined 3.6 percent to $20.6 million (64 cents a share) from $21.39 million (65 cents) a year earlier as revenues rose to $263 million from $243 million.

Nine-month earnings were $63.9 million ($1.97 a share) compared with $63.9 million ($1.94) in the comparable 1979 period. Revenues were $795.5 million versus $712.8 million.