Time Inc., which shut down The Washington Star in August and sold its plant to The Washington Post, yesterday reported third-quarter pretax operating income of $71.4 million, up sharply from $45.1 million in the same period last year.

"We had a very good third quarter," a Time Inc. spokesman said.

For the first nine months, operating income was $221 million on revenue of $2.38 billion, up from $179.1 million on revenues of $2 billion a year ago.

The giant publishing company's net income for the third quarter and for the year so far are both down from last year's figure, however, because of the one-time costs incurred in discontinuing operations of two subsidiaries. One was The Star, which Time abandoned after failing in an attempt to sell it, and the other was Time-Life Films, which Time sold to Columbia Pictures Industries.

Net third-quarter income, after the write-off from the discontinued operations, was $19.6 million (32 cents per share). In the same period a year ago, net income was $33 million (59 cents). The average number of shares outstanding in the quarter was 61.9 million, up from 56 million a year ago.

J. Richard Munro, president and chief executive officer, said that despite the drop in net income, "our third-quarter earnings from continuing operations this year were very strong," especially since last year's third-quarter figure was inflated by a $9.3 million after-tax gain on the sale of the Time & Life Building in Paris.

The company reported that in the third quarter the discontinued operations--the film division and The Star--had net after-tax losses of $19.5 million. For the first nine months, the loss was $35.7 million. The continuing after-tax losses were the reason cited by Time last summer for its decision to close The Star.

The figures do not include Time's gains from the sale of The Star's plant at 225 Virginia Ave. SE to The Post. The sale was completed in the fourth quarter, and the price has not been revealed.