Hechinger Co., a chain of 44 retail do-it-yourself home centers, yesterday announced a record 29 percent increase in both sales and profits for the third quarter ending Oct. 27, and its 39th consecutive quarterly earnings increase.

The Landover company reported sales of $100.4 million for the quarter, up from $78 million last year. Net earnings were $4.7 million (26 cents per share), compared with $3.6 million (20 cents) last year.

Sales and earnings were also at record levels for the first nine months, the company said. Sales for the period jumped 31 percent to $302.7 million, compared with $230.5 million a year ago. Net earnings climbed 33 percent to $14.8 million (81 cents) from $11.1 million (61 cents) last year.

Hechinger's first warehouse store, located in Raleigh, N.C., opened during the third quarter. Biospherics Inc., a Rockville research and development firm that does government contracting, yesterday reported revenue of $1.5 million for the third quarter ending Sept. 30, up slightly from $1.4 million in the same period last year. But the company reported a loss of $2,961 for the quarter, compared with profits of $71,528 (2 cents) last year, and blamed the losses on the costs of developing new products.

Total revenue for the first nine months of 1984 was $4.4 million, comparedwith $3.9 million for 1983. Nine-month earnings were $71,528 (2 cents), compared with $84,565 (2 cents) for the same period in 1983.

The company said it is producing oil pollution-prevention devices for the U.S. Navy and also is developing a diet sweetener.