OK! Exclusive: Mark Wahlberg Expecting Baby #3!

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OK! has learned exclusively that Mark Wahlberg, 36, and his fiancée of four years, Rhea Durham, 29, are expecting their third child later this fall!

As has become standard for many expecting celebs these days, the Boogie Nights star has yet to go public with the good news, although he has told friends he is having a son, due in September.

"Mark is thrilled to be having another baby," says one pal. "He is an amazing dad, and his two children are his world. He always used to joke about wanting at least a dozen kids, but it looks like he wasn't joking after all."

Mark and Rhea already have daughter Ella Rae, 4, and a son, Michael, 2. A source says that while the couple is getting along beautifully, Mark has always been—and remains— marriage-shy, and is in no hurry to walk down the aisle despite a news of a third child on the way.

For the complete story of Mark and Rhea's baby news, pick up the latest issue of OK!—on newsstands everywhere Thursday!