Jennifer Garner and Ben Affleck Welcome Third Child

UPDATED 02/28/2012 at 10:00 PM EST • Originally published 02/28/2012 at 07:20 PM EST

Jennifer Garner and Ben Affleck are proud parents for the third time.

The couple has welcomed a son in Santa Monica, a source confirms to PEOPLE. The new addition joins big sisters Seraphina, 3, and Violet, 6.

The couple announced they were expecting in August, with Garner telling Jay Leno in January that they knew the sex of the baby, but weren't sharing.

PHOTOS: Jennifer Garner's Preppy-Chic Maternity Style

"It would be so weird to have a boy," the actress told Ellen DeGeneres, but also said the experience would be "cool and different."

Affleck, in turn, said in 2010 that the three-against-one female-to-male ratio in his house leaves him significantly outnumbered.

In January, Garner and Affleck, both 39, were photographed walking arm-in-arm, with Garner sporting a sizable baby bump.

"He knows when to swoop in with the gesture. He's sweet that way," the actress says of her husband. "Honestly, I would do anything for that man, because I know it's not taken for granted."

RELATED: Jennifer Garner & Ben Affleck Name Their Son