Maryland Cup Corp. yesterday reported a "modest" increase in profits, but earnings were down at Allegheny Beverage Corp., the Richmond, Fredericksburg & Potomac Railroad and Owens & Minor, Inc., the Richmond drug wholesaler.

At Maryland Cup, the nation's biggest maker of paper and plastic food service products, net income for the three months ended June 30 climbed to $9.2 million ($1.36 a share) from $8.9 million ($1.33) a year ago. Sales rose to $183.8 million from $178.5 million.

For the first nine months of the fiscal year, Maryland Cup earnings climbed to $31.4 million ($2.47) from $30.8 million ($2.41), while sales increased to $470 million from $452.9 million."

Commented Maryland Cup President Samuel Shapiro: "Our earnings increase, though modest, must be considered noteworthy in view of generally decling corporate profits."

In contrast to the slight gain by the maker of Sweetheart cups, the local Pepsi-Cola bottler, Allegheny Beverage, said its net for the quarter ended June 30 slipped to $2.1 million (49 cents a share) from $2.5 million (58 cents) a year ago, even though sales climbed to $116.1 million from $112.8 million.

Chairman Morton Lapides said Pepsi sales increased, but at the expense of heavy promotional spending. Profits of the vending and food service division also were hurt by the cost of developing new customers to replace the business lost due to plant closings and layoffs.

Though it did not affect quarterly earnings, Allegheny disclosed that it had written off $1.8 million because of its stock in Wickes Cos. Inc., which filed for reorganization in bankruptcy in April. Allegheny tried unsuccessfully to take over Wickes just before Wickes went into bankruptcy court.

The Richmond, Fredericksburg and Potomac said its earnings for the first half of the year slipped to $7.8 million ($21.61 a share) from $8.4 million ($23.43) in the first half of 1981. Second-quarter earnings fell to $4 million ($11.22) from $4.5 million ($12.44) last year.

Rail freight revenues slipped to $12.3 million from $14.1 million for the quarter and were down 8 percent for the six months as a result of "the weak national economy," said President John Newbauer.

Owens & Minor reported second-quarter earnings of $404,000 (42 cents a share) compared with $877,000 (93 cents) in the same period last year, when profits were swollen by $426,000 as the result of an acquisition. Sales for the drug wholesaler increased to $51.2 million from $42.4 million for the three months ended June 30.

For the first half, Ownes & Minor earned $927,000 (97 cents) on sales of $98.7 million compared with $1.03 million ($1.09) on sales of $80.8 million last year.