Three of metropolitan Washington's major commercial banking companies - Riggs National Bank, American Security Corp. and Maryland National Corp. - yesterday reported increased profits during the fourth quarter of 1976.

Only one of the firms, however, reported higher earnings for the full year. American Security, holding company for American Security & Trust Co., said profits rose 5.2 per cent in 1976 to a record high of $16 million ($6.59 a share) compared with $15.2 million ($6.26) the previous year.

Indicated fourth-quarter profits were $1.99 a share compared with $1.71 in the same period of 1975, a growth that came as a surprise to some securities analysts who had forecast that American Security would find that American Security would find it difficult to match the strong 1975 performance.

Chairman Carleton M. Stewart, whose firm had shown a compound annual earnings growth of 9.5 per cent in the five years 1970-1975, attributed the gains last year to a restructuring aimed at meeting specific banking requirements of each business segment.

Deposits of AS&T, the District's second-largest bank, rose 6.5 per cent in 1976 to $1.15 billion on Dec. 31. Stewart forecast an "improved economy" in the coming year.

Riggs National Bank, the largest in Washington, reported that profits fell 10.2 per cent in 1976 to $13.2 million ($4.36 a share) compared with $14.7 million ($4.86) in 1975. Chairman Vincent C. Burkr Jr. said the decline reflected lower interest rates during last year as well as higher staff and operating expenses. The Riggs results include those of Central Charge Service, a wholly owned subsidiary.

In the fourth quarter, Riggs earned $3.9 million ($1.28), and gain of 21 per cent from $3.2 million ($1.06) in the same period of 1975. Burke said there was a higher average volume of loans and investments, including a higher level of tax-exempt securities, in the recent period.

Assets of Riggs rose 2.5 per cent during the year to $1.7 billion and deposits increased 8.3 per cent to $1.46 billion. During the five years ended 1975, Riggs earnings increased 7 per cent annually and total assets increased 10.1 per cent, also compounded annually.

Maryland National Corp., holding company for Maryland National Bank reported 1976 profits of $16 million ($2.24 a share) compared with a record $20.5 million (2.86) in 1975, a decline of 22 per cent which chairman Robert D.H. Harvey attributed to a large increase in reserves for loan losses in the second quarter (cutting earnings to 26 cents a share from 77 cents in the 1975 period).

In the fourth period, Marynat net income rose 6.7 per cent to $4.6 million (64 cents) from $4.3 million (60 cents). Assets rose 17 per cent in 1976 to $2.8 billion and deposits at the bank, largest in the state, climbed $100 million to the $2 billion level.

Bank analysts have forecast that commercial loan demands will begin to increase this year, leading to expanded earnings for such regional banking companies as Riggs, American Security and Marynat.