RCA Corp. reported a 28.8 percent gain in third-quarter revenue yesterday, as record sales rolled in from its electronics, NBC broadcasting and Hertz car rental units.

Philip Morris Inc. also had improved profits, while net income fell at American Airlines and Union Pacific Corp. rail company.

RCA Chairman Thornton F. Bradshaw said profits for the third quarter ended Sept. 30 climbed to $78.0 million (74 cents a share), compared with $60.5 million (53 cents) for the same period last year. Revenue totaled a record $2.47 billion, up from $2.27 billion a year ago.

Earnings for the nine-month period rose 56.6 percent to $238.2 million ($2.27) from $152.1 million ($1.23). Sales increased to a record $7.32 billion from $6.49 billion.

RCA's electronics unit gained substantially from the phase-down of its video-disc operations and from increased sales of color televisions, videocassette recorders, and new forays into the stereo TV market.

NBC posted record sales although earnings at both the television and radio divisions suffered from the costs of covering the election.

AMR Corp., the parent company of American Airlines, reported third-period profits of $78.6 million, almost 22 percent lower than the $100.7 million posted for same period last year.

The company said airline traffic was strong through the period, but discount fares cut into the carrier's yields. Nearly 50 percent of American's business is against low-fare carriers, forcing American to slash its own fares.

AMR, based in Grapevine, Tex., said its operating income for the period was $111.8 million, a $13.9 million drop from the same quarter last year. Revenue increased to $1.369 billion from $1.288 billion in 1983. Casey also said traffic improved 11 percent, but yield was down 6 percent.

Union Pacific Corp. saw its third-quarter profit fall 10.3 percent from a year earlier on losses in its refining operations.

The New York-based company said net income fell to $136.1 million ($1.10 a share) for the July-September period, from $151.7 million ($1.23) a year earlier. Revenue dropped to $2.03 billion from $2.26 billion.

William S. Cook, president and chief executive officer, said in a statement that the rail business "continued to be the strongest contributor to our overall earnings." He said Union Pacific Systems carried more carloads of automobiles, construction materials, coal, chemicals, steel and other freight.

But the company's profit was "penalized by losses in refining operations," as gasoline prices remained "unexpectedly weak," he said.

Philip Morris Inc. said record cigarette sales pushed its third-quarter profit up 15.4 percent from a year earlier.

The New York-based tobacco, beverage and packaged goods concern said net income rose to $321.6 million ($2.62 a share) from $285.9 million ($2.27) a year earlier.

Revenue rose to $3.67 billion from $3.46 billion.

Domestic sales of the company's cigarettes rose to record levels in the latest quarter, Hamish Maxwell, chairman and chief executive, said in a statement. International cigarette sales also improved, although their impact on earnings was adversely affected by the strong U.S. dollar relative to other currencies, he said.

Revenue was flat for the company's Miller Brewing Co. unit, although competitive pricing helped boost unit volume of Miller High Life, Lite, Lowenbrau, Milwaukee's Best and its other brands, he said.

Revenue rose at its Seven-Up Co. soft-drink subsidiary and the packaged goods division, Maxwell said, but revenue for the real estate division fell from a year earlier.

BankAmerica Corp., the nation's second-largest bank holding company, said its third-quarter earnings climbed 20 percent, but its nine-month profit slipped 11 percent.