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‘This Is Us' Season 3 Premiere Sneak Peek: Why Is Beth Swearing on Oprah Winfrey?

Get your first look at the third season debut of the hit NBC drama.

Would you swear on Oprah Winfrey? That's something Randall (Sterling K. Brown) asks his wife, Beth (Susan Kelechi Watson) to do in the exclusive sneak peek of the "This Is Us" season three premiere.

It's the Big Three's birthdays yet again and the Pearson family is all coming together under Randall's roof. It's a full house, Deja (Lyric Ross) is back with them, as is Zoe (Melanie Liburd), Beth's cousin, who is working on a documentary and staying with the family. As viewers know, Kevin (Justin Hartley) and Zoe eventually head to Vietnam, those events were witnessed in the season two finale flashforward, but what they don't know is Beth isn't on board with the new couple.

This Is Us Season 3: See Annie Leibovitz's Stunning Portraits

"Now I'm sure of it. Your brother and my cousin are knocking boots," Beth says in the exclusive clip above.

Video of a great white shark spotted off the coast of Cape May on June 21, 2014 was sent in to NBC10 from a viewer.

"Whatchu talking about, Beth?" Randall asks.

"It's an expression from the '90s. It means that they are having sex," she replies.

"I'm familiar with the expression, woman, I'm just asking why you're so positive," Randall says.

She just knows, and wants to kill Kevin in the face (her words), but Randall isn't so sure and doesn't want to ruin the birthdays.

Mandy Moore chats about getting to play in all the eras of "This Is Us" and why she's happy that she gets to spend a lot more time laughing in Season 3.

"It is my birthday and I need you to swear to it on Oprah," Randall says.

"I'm not doing that," Beth says.

But she relents. The swear on Oprah gets made. However, "Even Gayle lets Oprah down sometimes," Beth says behind Randall's back.

Expect to learn more about Beth this season, whether or not that includes what happens when you swear on Oprah remains to be seen. "We have some stuff coming for Beth right away early in season three where you really learn about her and where she comes from and what her background is that I think is going to be really exciting for the audience," series creator Dan Fogelman previously told E! News.

"This Is Us" returns Tuesday, Sept. 25 at 9 p.m. on NBC.

Milo Ventimiglia teases a younger Jack Pearson, Vietnam flashbacks and Jack and Rebecca's first dates in Season 3 of NBC's "This Is Us."

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