STSC Inc. yesterday announced an 11 percent increase in third-quarter profits, which rose to $530,000 (29 cents a share) for the fiscal quarter ended with February, up from $479,000 (28 cents) for the same quarter last year.

The Bethesda-based computer services company reported profits for the first three quarters of $1.33 million (74 cents), up from $1.071 million (63 cents) for the previous nine-month period.

STSCsales rose from $5.83 million for the third quarter in fiscal 1980 to $7.35 million for the same quarter during the current fiscal year. For the nine-month period, sales are up from $15.1 million to $20.49 million.

Solon Automated Services Inc., the District-based laundry services company, reported profits of $1.5 million (53 cents) for the quarter ended Dec. 31, up from profits of $1.36 million (48 cents) for the same quarter in 1979. Sales rose from $17.5 million for the 1979 quarter to $20.3 million last year.

American Furniture Co., a Martinsville, Va., company, reported a steep drop in first-quarter profits, which fell from $401,966 for the 1980 first quarter to $32,283 this year. Sales fell 8 percent to $15.4 million.