Jessica Simpson on Having Three Kids 7 and Under: 'There Is a Lot of Dividing and Conquering'

Jessica Simpson‘s house got a little busier on March 19 with the addition of a new baby girl, but the newly minted mom of three wouldn’t have it any other way.

The 39-year-old singer and actress says that while having three children age 7 and under is certainly difficult, she and husband Eric Johnson have learned how to tackle it together.

“Three kids is no joke. It is definitely constant, and the biggest challenge for me is trying to be present when I am pulled in so many directions,” she tells PEOPLE in this week’s issue. “They are all in such different phases now.”

The star’s oldest child, 7-year-old daughter Maxwell “Maxi” Drew, is embracing life as a social butterfly with a passion for animals, while son Ace Knute, 6, is “obsessed with sports,” Simpson says. Birdie Mae, meanwhile — like most 3-month-olds — keeps her focus on drinking milk and taking naps.

“There is a lot of dividing and conquering going on right now at our house,” Simpson reveals of parenting with Johnson, 39, who is a former NFL player.

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However, Simpson says that they make it a priority to take one night a week to themselves, handing Maxi and Ace off to Johnson’s parents and hiring a babysitter to take care of Birdie.

“We typically download a movie and order dinner or do something super low-key to just connect,” she tells PEOPLE. “Eric is the greatest motivator and connecting with him on morning walks is the most grounding part of my day.”

The fashion designer frequently offers fans glimpses of her home life on Instagram, showing off personal moments that feature things like Birdie’s adorable dimples and Maxi and Ace’s skills as older siblings.

“Birdie is so precious. She is a little smiley angel these days. It is the best thing in the world to see and feel her smile. It fills the room,” Simpson says.

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As if having a household full of children wasn’t enough, Simpson has even more on her plate, with a memoir for Harper Collins due out in February and the launch of JessicaSimpson.com, a sales destination for her Jessica Simpson Collection that doubles as a place for the star to share stories, ideas and photos.

“I think people will be surprised by a lot of what they will read,” Simpson says of her book. “I can’t wait to share it.”

For more on Jessica Simpson, pick up the latest issue of PEOPLE, on newsstands now.