The Washington Post Co. yesterday reported third quarter earnings of $7.9 million (51 cents a share), about $1.5 million less than profits for the same period a year ago.

The Post Company earned $9.5 million (59 cents) in the third quarter of 1978, but that included a $600.000 non-recurring gain from the sale of real estate.

The publishing and broadcasting company's revenues for the 13 weeks ended Sept. 30 increased to $137.5 million from $123.1 million a year ago.

For the first nine months of 1979, the Post Company reported earnings of $32 million ($2.03) on revenues of $420 million, compared with $33.8 million ($2.07) on revenues of $371.1 million last year.

The company's newspaper and magazine and book publishing divisions reported 11 percent increases in revenue for the quarter and its broadcasting division's sales were up by 15 percent.

Earnings for the two publishing divisions were down, the company said. The decrease was attributed to higher newsprint costs and other increased expenses in the newspaper division; to paper, postage and circulation cost increases at Newsweek magazine, and to the cost of producing a test issue of a new magazine called Inside Sports.

The company said the decrease in earnings per share was proportionately less than the overall earnings loss because of repurchases of stock -- 629,000 shares in the first nine months of 1979.

Union Trust Bancorp, a Baltimore-based bank holding company, announced third-quarter earnings of $2.4 million (97 cents a share), an increase of 27.3 percent over the $1.9 million (77 cents) earned a year earlier.

Nine-month net income rose from $6.1 million ($2.49) to $6.8 million ($2.75).

Chairman J. Stevenson Peck cited the company's strong loan activity as a reason for the increased earnings.

Overnite Transportation Co. of Richmond reported third-quarter net income rose to $4.75 million ($1.48 a share) from $3.8 million ($1.19) a year earlier. Revenues increased 15 percent to $58.5 million from $50.8 million in the third quarter of 1978.

Nine-month earnings were $13.2 million ($4.12) on sales of $175.5 million compared with $9.9 million ($3.10) on sales of $144.9 million for the same period last year. Revenues rose 19 percent to $172.5 million from $144.9 million.