Best Products Co. reported a $2.4 million loss for the Richmond company's third fiscal quarter ended March 28 compared with a loss of $4.2 million for the same period last year.

The company reported that third-quarter sales rose 18.8 percent from $64.4 million last year to $76.5 million.

In other reports, Planning Research Corp. a District-based consulting and engineering firm, has reported a 30 percent drop in profits for its fiscal third quarter ended March 31 to $747,000 (11 cents a share) from $1 million (15 cents) a year earlier.

Sales rose by 9 percent to a record $78.5 million.

Nine-month profits dropped to $964,000 (14 cents) from $4 million (56 cents) as sales rose from $199.1 million in 1980 to $232.2 million. The company attributed the drop to pretax charges of $4.3 million against second-quarter profits as a result of the closing of two nonprofitable businesses.

Arundel Corp. reported a loss of $1.8 million for the first-quarter compared with a loss of $1.2 million for the same quarter last year.

Federal Realty Investment Trust, a Chevy Chase investment company, reported a slight drop in first-quarter profits to $465,000 (24 cents a share) from $480,000 (33 cents) for the same period in 1980.

The company said its cash flow reached $963,000 the highest point in the company's history. Revenues rose to $3.6 million, a 46 percent increase over the same period in 1980.

Fair Lanes Inc. has reported record quarterly profits for the Baltimore company's third fiscal quarter ended March 31 of $2.2 million (50 cents a share) compared with $2 million (43 cents) for the same quarter in 1980. Sales totaled $15.49 million, up from $15.6 million last year.

Nine-month profits rose to $3.8 million (86 cents) from $3.6 million (86 cents).

Schwartz Brothers Inc., a Lanham retailer and wholesaler of music merchandize, recorded a slight profit for the year ended Jan. 31 of $2,035, negligible on a per-share basis compared with a loss of $147,710 for the previous year. Sales rose to $35.6 million from $30.5 million.

However, the company reported a 40 percent sales rise for the fourth quarter from $9.5 million during the previous year to $13.4 million.

Profits for the fourth quarter were $436,158 (54 cents a share) compared with $88,915 (12 cents) for the same quarter last year. Sales rose to $13.4 million from $9.56 million.