Despite the air controllers' strike, slumping travel business and a soft economy, Marriott Corp. increased its profits 12 percent for the three months ended Sept. 30 and posted a 23 percent gain for the first nine months of its fiscal year.

Marriott yesterday reported net earnings of $28.2 million ($1.05 a share) for the quarter, up from $25.6 million (95 cents) in the same period a year ago. Sales increased 15 percent to $503.4 million from $436.5 million.

For the nine months, Marriott earned $64.4 million ($2.40) on sales of $1.4 billion, compared with a profit of $55.1 million ($1.95) on revenues of $1.2 billion last year.

President J.W. Marriott said profits improved at all the company's major operations and said the "business performed very well given the obvious adverse conditions."

The controllers' strike cut air travel and directly hurt Marriott's contract food service, which provides airline food, and indirectly cut into the hotel business.

But contract food service profits increased 13 percent and hotel earnings were up 7 percent -- much less than the 26 percent gain in hotel revenues. Part of the hotel revenue increase came from adding 14 new properties with 6,100 rooms, which Marriott said are not yet contributing significantly to profits.

Restaurant profits increased 15 percent and sales were up 9 percent, even though Marriott's restaurants are serving fewer customers than a year ago. Even Marriott's Great America theme parks showed an improvement, with profits up 10 percent for the 1981 season.

Washington Gas Light Co. reported net earnings for the 12 months ended Aug. 31 increased to $16.2 million from $10.6 million, primarily because of higher utility rates. After paying preferred stock dividends, earnings per common share increased to $3.47 from $2.42. Total revenues for the utility climbed to $511.6 million from $418.3 million.