Chris Pratt and Katherine Schwarzenegger Expecting Baby No. 3

Chris Pratt and Katherine Schwarzenegger are expanding their family ... because she's got another bun in the oven.

Arnold Schwarzenegger's daughter is pregnant with the married couple's third child ... TMZ has confirmed.

Unclear how far along Katherine is in this pregnancy ... ditto for whether it's a boy or a girl on the way.

For what it's worth, Chris and Katherine have been to some public events recently and there was no noticeable baby bump.

We actually talked to Chris Thursday in L.A., but he didn't let on about the new addition on the way -- instead, he told our photog he's cool with making the jump from Marvel to DC ... if his director pal James Gunn has a role for him in the DCU films.

As you know, Katherine and Chris already have 2 young daughters together -- 2-year-old Lyla Maria and 13-month-old Eloise Christina. Chris is also father to 10-year-old Jack, who he had with his ex-wife, Anna Faris.

PEOPLE was first to report the new pregnancy.

So, this makes 3 pregnancies in as many years for Katherine and Chris, and he's about to be a father to 4. Something tells us Elon Musk and Nick Cannon would approve.