CBS News boss Wendy McMahon is putting her mark on “CBS Mornings” — just weeks after Norah O’Donnell’s surprise exit from the network’s “Evening News” program, The Post has learned.

The third-place morning show, co-hosted by Gayle King, Tony Dokoupil and Nate Burleson, will launch a third hour in select markets and on the CBS News 24/7 streaming channel late next month in a fresh bid to compete with its No. 1 and No. 2 rivals, ABC’s “Good Morning America” and NBC’s “Today.”

According to sources, Dokoupil and “CBS Weekends” host Adriana Diaz are expected to become the new co-hosts of the third hour, which will broadcast live from 9 to 10 a.m. ET weekdays from the “CBS Mornings” studio in Times Square.

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Dokoupil is expected to continue his anchoring duties on “CBS Mornings,” said a source with knowledge.

The new show will likely be televised in a handful of CBS markets, including Los Angeles and Chicago — but not in New York, sources said.

CBS declined to comment.

In a memo obtained by The Post, Shawna Thomas, executive producer of “CBS Mornings,” told staffers Friday that she would be producing the third hour with her team amid much speculation.

“One of the worst kept secrets at CBS News is true. A few of our colleagues and I have been working on developing a third hour of ‘CBS Mornings’ that we think will hit the airwaves at the end of September. It is being developed for a few of our owned stations and CBS News 24/7,” she wrote.

Thomas said the show “won’t look or feel exactly like the first two hours,” and that the talent and the name will be different, but “it will still be the smart and engaging content everyone expects from the ‘CBS Mornings’ family.”

Thomas said the show is looking to hire producers even as CBS’ parent company, Paramount Global, is slashing $500 million from the budget ahead of its expected merger with Skydance Media next year.

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The move comes as CBS News CEO McMahon is pushing to amp up viewership across TV and streaming for the struggling news division.

Recently, McMahon shook up the last-place “CBS Evening News,” replacing anchor Norah O’Donnell with CBS correspondent John Dickerson and WCBS anchor Maurice DuBois under “60 Minutes” boss Bill Owens, who will overhaul the show’s format to include more in-depth reports.

Unlike “Evening News,” however, “CBS Mornings” is experiencing some ratings growth, closing the gap in the advertising-coveted 25- to 54-year-old demographic with ABC’s “GMA,” in particular.

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“It is a smart move and natural evolution of a growing brand,” said a CBS insider. “‘CBS Mornings’ is capitalizing on the momentum and edge on ‘GMA’ that they’ve seen this season to continue growing and building.”

In the second quarter of the year, “CBS Mornings” trailed “GMA” by just 68,000 viewers in the demo. This comes as the show outperformed “GMA” 15 times in the 25-54 demo, Nielsen said.

Overall for the quarter, CBS reeled in 2.1 million total viewers and 426,000 viewers in the demo.

It trailed ABC’s “GMA,” which had 2.8 million total viewers and 494,000 demo viewers.

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NBC’s “Today” grabbed 2.7 million total viewers and 620,000 viewers in the demo.

The tightening morning show race comes as ABC is searching for a president of its news division, which oversees “GMA,” “World News Tonight” and other marquee shows.

The network ousted embattled ABC News president Kim Godwin in May amid a slew of missteps and softening ratings at the lucrative morning show.