THE PARENTS: Suzy Graybill, 31, and Donny Graybill, 30, of Wyndmoor

THE KIDS: William Donald, 5; Ellie Catherine, 3; Lily Elizabeth, born June 29, 2018

A NON-PARENTHOOD CHALLENGE: Training for a half-Ironman triathlon in June. They take turns swimming at the Y, they bike on stationary trainers at home, and they run with the kids in jogging strollers.

Her water broke at 29 weeks. Not a gush, but a trickle. Enough for the OB to order four weeks of hospital bed rest and antibiotics. And two weeks into her stay at Harrisburg Hospital, Donny left for a three-month program at Navy Officer Candidate School in Newport, R.I.

For Suzy, a self-described micro-manager who rarely sits still, it was a restless time. “I was insanely anxious,” she says. “I missed Donny. It was uncharted territory, becoming a parent.”

William arrived on Sept. 8, 2013. His father had wrangled a two-day leave. “He’s a good-looking guy,” Donny said. “He’s not quite a guy yet,” Suzy answered.

The next morning, Donny headed back to Newport. Suzy was recovering from a C-section; William remained in the hospital’s NICU for 18 days. It was — so far — the most harrowing stretch of their partnership.

“As crazy as it sounds, it did make us stronger,” Suzy says. “For Donny, having to leave his wife and his baby, who was struggling: It seemed counterintuitive. Nobody wants to have to make that choice. Everyone does hard things; life is hard. But that was our hard thing.”

They’d met by happenstance — a party at Shippensburg University hosted by one of Donny’s track teammates. A week later, she spotted him sitting alone in the cafeteria and offered her phone number with an invitation: Come to my house and watch a movie sometime.

Donny was struck by her confidence. He didn’t realize the invitation meant all of Suzy’s roommates would be watching the movie with them.

“I felt an instant connection with him,” Suzy recalls. “I thought: This is something that could last.” Donny had focus and drive; he studied and ran track and held a part-time job in the university’s sports medicine clinic. As for Donny, “I knew even before she graduated that it was turning into something more than a regular relationship.”

He proposed with the help of a tree, a hefty pine that stood along the main trail leading up from Donny’s family’s cabin in Tioga. He carved their initials into it, along with an image of a ring and the date — October 21, 2011 — inside a heart.

“Hey, what’s on that tree over there?” he said as they returned from a hike. Suzy looked, then spun around; Donny was on one knee. They married the following July. “Most people say their wedding day was a blur, but I remember everything,” Donny says: the Mass, the exchange of rings, the drive to the reception in his grandmother’s pink ’57 Chevy.

Having children was never a question. Suzy recalls riding the bus to school with her little sister, who was just starting kindergarten, and saying, “I want you to sit on my lap in case you get scared.” She babysat enthusiastically; later, she became a teacher. As a couple, the two would watch kids playing in a kind of silent accord: We’ll be good parents.

But Suzy, who had been diagnosed with a uterine abnormality in her early 20s, worried that conception and pregnancy might be difficult. The first part happened easily — she was grinning over a positive test six months after their wedding—but carrying to full term proved hard. With William, she needed bed rest; with the others, progesterone injections to prevent premature labor.

Meanwhile, a military life, though familiar from Suzy’s own childhood, meant nomadic hopping from Newport, where they moved after Donny’s stint in Officer Candidate School, to Hampton, Va. That’s where Ellie was born, at 35 weeks, a healthy baby with startlingly blue, almond-shaped eyes.

Though the family was together, the transition from one child to two felt like a giant stretch. The kids were just 22 months apart; in the morning, both would be on Suzy’s lap, sobbing because each wanted to be alone with her. “I felt very divided with two. I thought: I cannot be everything to everyone here.”

Her solution was to nourish the sibling relationship. “Say good morning to your sister,” she would remind William. “Say good morning to your brother. If you two are good to each other, you’ll have a friend through your whole life.”

The family moved to Philadelphia in November 2016. “I thought I was lucky,” Suzy says. “Blessed. I had two healthy kids, a boy and a girl. Why would I push things? But I got this feeling like I wanted another baby. And Donny said yes.”

He was scheduled for another deployment, a three-month medical mission in Central America, a region where Zika was prevalent. If they were going to conceive, the doctor advised, they should do so before Donny left.

They found out on Halloween, but waited until January to show an ultrasound image to William and Ellie. “Do you know what this is? It’s your little sister,” Suzy told them. William looked puzzled: Wasn’t Ellie his little sister? Ellie lifted her mother’s shirt and stared: “Where? I want to see.”

Once again, Suzy managed a portion of her pregnancy — this time, with two toddlers in the mix — on her own. Donny could usually FaceTime from the tent cities where he was posted in Guatemala and Honduras. And he was home by the time Lily was born, at 37 weeks.

Now, the hardest moments come when all three kids are tugging in different directions. “The baby’s crying, the 3-year-old needs a snack and my 5-year-old is trying to have a conversation with me about an endangered species,” Suzy says. But those are tempered by tranquil interludes: family walks, or the pre-dawn moments in bed when the older kids pass their adored baby sister back and forth.

“It’s easy to love your child,” Suzy says. “It’s not even that hard to do everything you have to do to provide. The hardest thing is being the best version of yourself. They’re looking at you, at every moment.”