MCI Communications Corp., the Bell System's largest long-distance telephone competitor, posted record earnings during its third quarter on a 110 percent increase in revenue, the company announced yesterday.

Net income climbed 64 percent to $43.6 million (44 cents) for the three months ending Dec. 31, 1982, compared with $26.6 million (28 cents) during the same period last year.

The increase in third-quarter revenue, also a record for the company, raised the 1981 total of $140.6 million to $295 million.

MCI said the third-quarter earnings also led to new records for revenue and earnings for the first nine months of its fiscal year, which ends March 31.

Revenue for the nine months totaled $741.8 millon compared with $341.6 million during the year-earlier period. Earnings climbed from $52.7 million (54 cents) to $117.6 million ($1.20) in 1982. Per-share earnings reflect a 2-for-1 stock split in September, and prior earnings were restated to take that into account.

William G. McGowan, MCI's chairman and chief executive officer, said the favorable balance sheet reflects "continuing strong demand for the company's services in both the business and the residential sectors."

McGowan said MCI's position as AT&T's oldest and largest long-distance competitor has been helped by recent Federal Communications Commission decisions.

The FCC ruled Dec. 22 that telephone companies gradually will have to abandon their traditional practice of subsidizing local telephone service through inflated long-distance charges. "Because of MCI's superior efficiencies in the use of capital, people and technology, we are a clear beneficiary of this policy," McGowan said.

The company's introduction of a service known as Omni-Call last summer contributed to higher usage of its long-distance network by customers, McGowan added. Omni-Call allows MCI customers to place a long-distance call anywhere in the United States, even if MCI's own network has yet to reach that city. The service relies on leased facilities from the American Telephone & Telegraph Co. to augment its own network.

MCI is the oldest and largest of a handful of companies competing with AT&T for long-distance telephone traffic. It has constructed an extensive microwave long-distance network across the country and promises its roughly 1 million customers significant savings on interstate calls.

The company also has diversified over the past year, acquiring Western Union International Inc. from the Xerox Corp. Western Union International is one of the country's largest so-called record carriers, handling telex and data transmissions to overseas points. The acquisition of Western Union International also brought to MCI extensive paging and mobile telephone holdings.