The Du Pont Co. yesterday reported a third-quarter net income of $278 million that included two months of earnings from Conoco, Inc., its newly acquired petroleum subsidiary.

Third-quarter 1981 earnings per share were $1.44 compared with 57 cents per share for the same time last year. Sales in the third quarter totaled $7.5 billion compared to 1980 third-quarter sales of $3.19 billion.

Du Pont's net income in the third quarter of 1980 was $90 million. Du Pont acquired Conoco this summer in the largest takeover in business history.

Since Aug. 1, Conoco sales accounted for $3.48 billion of the increase in Du Pont sales for the third quarter compared with the comparable period last year.

Conoco's contribution to Du Pont's consolidated third-quarter net income was $114 million after adjustments were made for purchase accounting and eliminating minority interest, the company said. Because of the adjustments and because only two months of Conoco earnings were included, Conoco's income is not comparable with results reported by Conoco in prior periods, the company added.

After-tax interest expense on new loans related to the acquisition of Conoco reduced current-quarter net income by $58 million.

Sales in the first nine months of 1981 totaled $14.8 billion compared with $10.2 billion during the same period last year, excluding the cumulative effect of a required change in accounting method for investment tax credit.

Nine months' net income was $753 million or $4.46 a share compared with $529 million or $3.37 a share for the same period last year.

The chemical company said that in accordance with generally accepted accounting principles, prior period results have not been restated because the acquisition of Conoco was accounted for by the purchase method.

Superior Oil Co. said its third-quarter profit rose 3.4 percent from a year earlier while revenue climbed 33 percent.

Superior said it earned $80.8 million, or 64 cents a share, in the quarter ended Sept. 30, compared with $78.1 million, or 61 cents a share, a year earlier.

Third-quarter revenue rose to $499.8 million from $376.1 million.