The Aluminum Company of America yesterday reported a slight increase in 1984 third-quarter sales and operating revenue over the same period last year.

Sales and operating revenue for the 1984 third quarter were $1.43 billion, compared with $1.39 billion a year earlier, Alcoa officials said.

For the first nine months of 1984, sales and operating revenue were $4.36 billion, up 15 percent from $3.79 billion for the comparable 1983 period, officials said.

Third-quarter earnings for 1984 were posted at $60.1 million (74 cents a share), compared with $57.7 million (72 cents) a year earlier, company officials said.

Alcoa earned $270.8 million ($3.32 a share) for the first nine months of 1984, compared with 1983 earnings of $78.0 million (96 cents), officials said.

St. Regis Corp., a leading forest products company, said its third-quarter profit surged 150 percent from a year ago, with more than half of the increase coming from the sale of assets.

St. Regis earned $49.7 million ($1.50 a share) in the three months through September, compared with $19.4 million (48 cents) in the third quarter of 1983. Revenue fell 3.5 percent to $703 million from $729 million.

St. Regis said the results included a one-time gain of $117 million from the sale of assets, which was partially offset by $85 million in nonrecurring charges associated with the company's proposed acquisition by Champion International Corp.

For the first nine months of 1984, net earnings nearly tripled to $116 million ($3.27) from $42.1 million ($1.11) the year before. Revenue rose 16 percent to $2.43 billion from $2.09 billion.

The company said third-quarter operating income grew for all but the construction products unit.