MCI Communications Corp. yesterday reported a 46 percent increase in third-quarter fiscal year profits, giving the District-based long-distance telephone firm a 61 percent profits rise for the first three quarters.

The company reported profits of $5.1 million (4 cents a share) for the quarter, which ended on Dec. 31, compared with profits of $3.5 million (1 cent) for the same quarter last year. The company said profits before taxes for the quarter were $5.6 million, the hghest quarterly profits figures in MCI's history.

For the nine-month period, the company reported profits of $13.2 million (13 cents), compared with profits of $8.1 million (10 cents) for the same period in 1979.

Revenues rose to $61.6 million for the third quarter, a 63 percent rise over third-quarter profits of $37.8 million for the same quarter in 1979. For the three quarters, revenues rose 61 percent to $162.4 million.

"We are not into what has historically been MCI's strongest quarter," said MCI Chairman William McGowan. "The results of the three months ended December 1980 have added substantial momentum to our progress for the full fiscal year."

Two more area banks reported record earnings for 1980 yesterday.

National Savings and Trust Co. reported year-end income before securities transactions of $4.704 million ($10.28 per share) compared with $4.054 million ($8.86) in 1979, and NS&T directors announced a 20-cents-a-share increase in the bank's dividend.

NS&T Chairman Joseph H. Riley announced that 16 percent earnings increase and a regular quarterly dividend of $1.20 per share on the bank's capital stock, par value $10. The dividend is payable Feb. 2 to stockholders of record at the close of business Friday, Jan. 23., the bank said. Riley also said that net income had increased this year compared with 1979.

Deposits at the end of 1980 were $406 million compared with $431.1 million in 1979. Assets declined to $512.3 million compared with $527.5 million a year ago.

Unitted National Bank of Washington reported earnings before taxes and securities transactions of $783,127 ($3.18 a share), which the bank said was an increase of 39.1 percent. Net income rose to $423,170 ($1.72) compared with $251,000 ($1.11) in 1979, an increase of 68.6 percent.

On Jan. 2, the bank paid an additional 1980 dividend of40 cents per share. The bank had already paid a dividend of 10 cents per share in the first quarter of last year.

Total deposits rose from $52.1 million at the end of 1979 to $58.5 million at the end of 1980. Total assets increased to $62.7 million from $56.2 million. Net loans in 1980 were $27.9 million compared with $25.3 million in 1979.

Computer Data Systems Inc. reported a 72 percent increase in net income for the six months ended Dec. 31. Income for the six months period in 1980 was $525,500 (90 cents a share) compared with $304,700 (55 cents) for the same period in 1979.

Income from continuing operations was $507,700 compared with earnings of $288,400 in the same period in 1979. Revenues also increased, rising to $8.335 million -- an increase of 24 percent over the $6.711 million revenues recorded for the same period in 1979.