Three leading communications corporations have reported record sales and profits during 1978.

Los Angeles-based Times Mirror Co., the largest publicly traded newspaper publisher, reported yesterday an increase of 30 percent in operating profits for the year with newspaper, television and magazine properties posting "continued success well beyond our expectations," said Chairman Franklin Murphy. Counting a one-time property sale gain, Times Mirror earnings were up 48 percent.

The Washington Post Co., meanwhile, said its profits last year increased 28 percent, not counting one-time gains from sales of a radio station and real estate. And Knight-Ridder Newspapers Inc., largest American newspaper publisher in terms of daily circulation, posted a 25 percent earnings gain.

Times Mirror earnings totaled $142.4 million ($4.13 a share) compared with $96.1 million ($2.77) in 1977, as revenues jumped about 25 percent to $1.43 billion. The 1978 profits included $17 million (50 cents a share) from the sale of 69,000 acres of timberland in Northern California.

The California firm publishes the Los Angeles Times, Newsday on Long Island, the Dallas Times Herald, magazines and books and is engaged in forest products, cable television, broadcasting and art products.

During the fourth quarter, Times Mirror earned $47.8 million ($1.40 a share) compared with $25.5 million (74 cents) as revenues increased to $363 million from $313 million. The nonrecurring gain from timberland sales accounted for 50 cents of per-share profits in the recent period.

Murphy said pre-tax newspaper profits rose 42 percent with advertising lines up 16 percent. Pre-tax profits from newsprint and forest products rose 3 percent, reflecting the impact of a 3 1/2-week newsprint mill strike. Newspapers accounted for $622 million of overall revenues while forest products were at exactly half that level -- $311 million.

Total earnings of the Washington Post Co. last year were $49.7 million ($3.06 a share) compared with $35.5 million ($2.09) in 1977, a gain of 40 percent. Included in the 1978 figures are $4.3 million of nonrecurring gains -- mainly from the sale of WTOP radio to Outlet Co. last summer. Revenues increased by 19 percent to $520 million.

In the fourth quarter alone, the Washington firm reported profits of $16 million (99 cents a share), up 23 percent from $13 million (79 cents) in the 1977 period as revenues increased 18 percent to $149 million.

The Post Co. said revenues and pre-tax profits increased in each of its three major divisions -- newspapers, magazines and books, and broadcasting. Revenues from magazines and books and broadcasting. Revenues from newspapers -- The Post, Trenton (N.J.) Times and Everett (Wash.) Herald -- rose 22 percent as pre-tax profits increased to $38 million from $27 million. Advertising lines in The Post were up 6 percent over 1977.

Magazine and book revenues rose by 18 percent with advertising revenues at Newsweek, the principal subsidiary, up by 21 percent. Pre-tax profits from the magazine and book business increased to $30.7 million from $26.6 million. Broadcast revenues rose 15 percent as pre-tax profits increased to $20.7 million from $17.6 million.

Earnings also increased last year at a Canadian newsprint firm owned 49 percent by the Post Co. and at the International Herald Tribune of Paris, in which the Washington company has a one-third interest.

Miami-based Knight-Ridder profits last year totaled $76.8 million ($2.34 a share) compared with $61.2 million ($1.85) in 1977, as revenues rose to $879 million from $752 million. Fourth-quarter earnings were $23 million (70 cents a share) vs. $19.4 million (60 cents) as revenues increased to $243 million from $210 million.

President Alvah Chapman said fullrun ad linage in the company's 32 dailies rose 7 1/2 percent. Knight-Ridder papers include the Philadelphia Inquirer, Detroit Free Press, San Jose Mercury, Miami Herald and the Journal of Commerce. Broadcast properties accounted for $20 million of 1978 revenues.