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Mama knows best!

Denise Richards kept the adoption of her third daughter "extremely quiet," a source tells Us Weekly. The process of adopting Eloise Joni -- named after Richards' mother, who passed away in 2007 -- "has obviously been going on for quite some time."

Richards, 40, has two daughters -- Sam, 7, and Lola, 6 -- from her previous marriage to Charlie Sheen, 45.

So what prompted the Wild Things actress go public with the news of her third daughter? "At some point, they wanted to leave the house!" the source says, adding, "The whole family is thrilled. Denise is a fabulous mother and the girls are great big sisters."

"Thank you for all your sweet messages," Richards tweeted Wednesday. "The girls and I are over the moon, and so is my dad, Grandpa Irv."