CBS Inc.'s third-quarter earnings decreased to $54.3 million ($1.95 a share) from $55.7 million ($2) last year, although profits of its CBS broadcast, records and publishing groups rose and overall revenues increased to $995.1 million from $951.1 million, the entertainment company reported yesterday.

CBS attributed its earnings decline to "sharply higher" costs in development of its new cable television, home video and theatrical films divisions and to currency exchange losses in complying with the Financial Accounting Standards Board rules.

Third-quarter earnings also were affected by a pretax loss of $2.8 million on the disposition of two small business operations compared with a $7.5 million profit in the 1980 quarter on the sale of two Canadian cable television companies.

Nine-month earnings were $110.3 million ($3.95) on revenues of $2.99 billion compared with earnings of $123 million ($4.41) on revenues of $2.89 billion in the like period a year ago.

Aluminum Co. of America said yesterday that its third-quarter net income was $81.3 million ($1.09 a share), up 8 percent from the same 1980 period, on the strength of a one-time gain of $9.5 million from a land sale. Revenues totaled $1.28 billion.

Earnings for the 1980 third quarter were $74.7 million ($1.03) on sales of $1.26 billion a year earlier.

For the first nine months of the year, Alcoa earned $264 million ($3.55) on sales of $3.89 billion. In 1980, earnings for the same period were $367 million ($5.14) on sales of $3.85 billion.

General Telephone & Electronics Corp., the nation's second-largest utility, said its earnings rose 11 percent to $159.7 million (94 cents a share) in the third quarter from $143.9 million (89 cents) in the same 1980 period as revenues also rose 11 percent, to $2.73 billion from $2.46 billion

Results for the year-earlier period did not include a $64 million loss taken in connection with the company's sale of part of its consumer electronics business and preparations to sell the remaining portions of that business.

R.J. Reynolds Industries Inc., a diversified company with interests in tobacco, shipping, energy, packaging and foods, said that third-quarter earnings rose 9.3 percent to a record $203.5 million ($1.87 a share) from $186.1 million ($1.70) a year ago.

The company said sales rose 11 percent to $2.9 billion from $2.6 billion in the 1980 third quarter.

Nine-month earnings rose 18 percent to $590 million ($5.41) from $533.5 million ($4.89) as sales were up 14 percent to $8.6 billion from $7.5 billion.

Buoyed by record domestic cigarette sales and gains from its beer and soft drink businesses, earnings for Philip Morris Inc. surged 20 1/2 percent in the third quarter to $202.1 million.

Philip Morris Chairman George Weissman reported yesterday that sales and earnings for both the third quarter and nine months set records.

Earnings for the third quarter came to $1.62 a share on revenues of $2.9 billion, up from profits of $167.6 million ($1.34 a share) on revenues of $2.7 billion in the same 1980 period.

Nine-month earnings rose 21.6 percent to $536.7 million ($4.30) on revenues of $8.3 billion compared with earnings of $441.4 million ($3.54) on revenues of $7.4 billion a year earlier.

Allied Corp., formerly known as Allied Chemical Corp., reported yesterday that its third-quarter profits were $80 million ($2.02 a share), up 21 percent from the $66 million ($1.80) earned a year earlier. Revenues rose to $1.544 billion from $1.323 billion.

The net income does not include an extraordinary gain of $38 million from the sale of Canadian oil and gas properties.

For the nine months, Allied earned $251 million ($6.75) before the extraordinary gain on revenues of $4.711 billion compared with $207 million ($5.88) a year earlier on revenues of $4.071 billion.

American Cyanamid Co., a giant chemical manufacturer, earned $42 million (88 cents a share) in the third quarter, up from $40.2 million (84 cents) a year ago, on a rise in sales to $935 million from $869.2 million.

Nine-months profits were $131.3 million ($2.74) on sales of $2.722 billion compared with 1980 nine-month profits of $110.8 million ($2.31) on sales of $2.536 billion.

Chairman James G. Affleck said fertilizers and other agricultural chemical sales were about even with a year ago but earnings from these products were down. Medical product sales were higher but earnings also were off. General chemical sales, consumer products including toiletries and men's fragrance and sales of Formica decorative laminates produced better earnings.

Reynolds Metals Co. said weakness in the economy and lower margins pushed net income down to $29.5 million in the third quarter of 1981 from $36.6 million a year earlier, continuing a year-long trend of lower profits.

Earnings per share fell to $1.50 from $1.88, while net sales were lower at $906 million compared with $931.7 million.

The company said net income fell to $107.8 million in the first nine months from $146.8 million for the first nine months of 1980.

Declining rail traffic and high interest costs slashed third-quarter profits of Southern Pacific Corp. to $1.36 a share from $1.99 a year earlier in spite of a rise in revenues to $828.44 million from $727.43 million.

Net income slid to $36.71 million from $53.48 million.

Nine-month profits rose to $104.23 million ($3.86 a share) on revenues of $2.463 billion from $91.57 million ($3.40) a year earlier on sales of $2.106 billion.

TRW Inc., a high-technology manufacturer, said it earned $83 million ($2.42 a share) in the third quarter, up nearly 70 percent from $48.9 million ($1.45) in the same 1980 period, mainly because of a one-time gain of $30.4 million from the sale of some assets. Without the extraordinary gain, earnings would have been up 7 1/2 percent to $52.6 million.

Sales for the period were $1.29 billion, up 8 percent from $1.19 billion in the 1980 quarter.

For the first nine months of the year, TRW earned $201.9 million ($5.88) compared with $154.7 million ($4.70) in the comparable 1980 period. Sales were $4.01 billion compared with $3.7 billion.