Jennifer Garner is a mom three times over as of last February, when her son Samuel was born, joining sisters Violet (6) and Seraphina (3). But she’s still most at home playing women who don’t have a clue about motherhood.

“NOBODY is prepared for this,” she says with a laugh. “And sometimes, the harder you try, the more you mess up. There’s nothing that will humble you more. I am constantly feeling that I’ve screwed something up, or will screw something up. You learn as you go along.”

Learning to be a mom has had fringe benefits for her as an actress. Garner earned some of the best notices of her career when she played a woman who can’t have a child in “Juno,” work that Entertainment Weekly called Oscar-worthy and “unbelievably touching.” In “Juno,” her character and her husband plan to adopt the child of a pregnant teenager.

In her new film, “The Odd Life of Timothy Green,” Garner plays another woman in search of a baby she cannot have. And this time, she and her husband (played by Joel Edgerton) find a little boy quite literally in the cabbage patch.

“It’s as honest depiction of parenthood as you ever want to see,” she says. “Aside from finding a boy in the garden, this is about as true as you’re going to get — faking your way through parenting, making it up as you go. You fake it ’til you make it, right?”

Right.

Of course, Garner, 40, and husband Ben Affleck have access to all the child-rearing help Hollywood royalty can afford. She took 16 months off before and after Samuel’s birth. And on returning to work, she found the “mothering gene” still switched on as Cindy, mom-in-training to a magical little boy in “Timothy Green,” which opens Wednesday.

CJ Adams has the title role in the film, a boy of 10 who springs up out of the garden to fulfill his parents’ longing, their hopes for a child and what he would be like. And having a little boy on the set meant extra duty for Garner.

“I was like, ‘Do you need a snack? It looks like your energy’s failing.’ ‘Do you need some water? Remember to hydrate.’ ‘Do you need to go to the bathroom? It’s been a while.’ ‘Do you need to be covered up? Are you cold?'”

Garner laughs.

“Acting, as ‘cush’ of a job as it usually is, has long periods of being very uncomfortable, which is a lot to ask of a little kid,” she says. “They have to stay focused through all that discomfort, too. CJ was covered in mud that’s dry and itchy, or was being wet-down all the time so that the mud has the right look to it. Pretending to be cold when it’s really hot — 100 degrees — and you’re dressed in all these itchy winter clothes. Nothing an adult should be complaining about, but for a little kid, your heart goes out to him.”

Actors create and break-up families with every film shoot, but Garner says that’s harder to do when there are child actors involved. First of all, she wanted to take CJ home with her.

“What an adorable kid! I got so attached to him that I couldn’t bear saying goodbye to that boy. I mean, I’ve got three of my own. But I couldn’t seem to move on. I’d be talking with him on the phone, texting with his mom. ‘The next time I’m going to see you is…'”

That probably doesn’t happen to every actor who works with children. But Garner’s reality is a lot more down-to-Earth than Hollywood. The Charleston, W.Va., native is in Charleston as we speak. “I am still a small-town girl. I am walking up and down the driveway of the house of my best girlfriend growing up. We’re still very close. Our kids are napping. She’s sipping iced tea, and I’m out here, listening to the leaves rustling overhead, the birds, doing interviews.”

That’s another trait she’s most at home carrying off on the screen — the “small town girl.” Garner may have gained fame as a rear-end-kicking TV secret agent (“Alias”) for J.J. Abrams, and played her share of society types (“Arthur”) and masked vigilantes (“Elektra”).

But “there’s nothing better than knowing that you have some place you belong and have always belonged. I like it about my life, and I like it in the characters I play, too.”