Suburban Bancorporation yesterday reported its third consecutive quarter of lagging 1980 profits, raising the prospect that the largest bank in the Maryland suburbs will fail to match its solid 1979 performance.

Suburban, the Hyattsville-based holding company, reported third-quarter profits of $3.9 million (83 cents a share), down from the $4 million (85 cents) in profits the bank reported for the third quarter this year.

For the first nine months of the year, the bank reported net profits of $12 million ($2.54), compared with profits of $12.3 million ($2.61) for the same period of 1979.

Although Suburban's net interest income increased by $250,000 to $17.2 million during the third quarter, the provision for loan losses rose by $398,000 to $2 million for the quarter.

Yet, despite the flat profits picture, Suburban's deposits rose by 4.8 percent to $1.2 billion, and average total assests of the corporation rose by $4.9 percent to $1.4 billion.

Robert Tardio, the chairman of Suburban, said the earnings performance was due to slow growth in net interest income because of narrowed interest rate spreads, economic uncertainties and higher operating expenses.

In another report yesterday, First American Bank of Maryland reported net profits of $609,000 (61 cents a share) for the third quarter, down from profits of $683,000 (66 cents) for the same quarter last year.

For the first nine months of the year, profits at First American, an affiliate of Financial General Bankshares, are down sharply, falling to $1.6 million ($1.57) from $1.7 million ($1.76) for the same period last year. Assest are also down, as of Sept. 30, dropping from $295.9 million in 1979 to $285 million on that date this year.

D.C. National Bank has reported a 37 percent increase in third-quarter profits. D.C. National reported profits before securities transactions of $488,000 ($2.38 a share), up from $357,000 ($1.74) for the third quarter of last year.

D.C. National, the city's seventh-largest bank, also reported a 44 percent increase in profits for the first nine months of the year, as profits rose from $4.49 a share last year to $6.47 a share this year. Average deposits for the first nine months rose by 14 percent over last year to $97.7 million.

In another report, Madison National Bank reported a 22 percent third-quarter profits rise, as profits rose to $1.8 million ($4.27 a share), compared with $1.4 million ($3.50) for the same period last year.

Madison National's total asets increased from $151.9 million at the end of the 1979 third quarter to $166.1 million at the end of the same quarter this year. Deposits rose to $135.7 million at the close of the quarter from $132.5 million at the end of the same quarter in 1979.