3 women share how they collectively shed more than 400 pounds

— -- Three women who lost more than 120 pounds each shared their weight-loss stories and secrets of their success.

Melody Perdue, 33, who, like the other women, is featured in People magazine's new “Half Their Size” issue, once weighed more than 300 pounds. She credited a desire to be more active with her son for her 180-pound weight loss.

"I was sick and tired of being immobile and just being lazy and laying around," Perdue said on "Good Morning America." "I definitely wanted to be there for him and to be active with him and do everything that I couldn't do as a child."

Perdue, of Franklin, Massachusetts, said she has struggled with weight most of her life and tried dieting but would ultimately "gain the weight back."

After a near-fatal car accident in 2012, Perdue recalled she was shocked to discover she weighed 283 pounds.

"I just remember being immobile," she said. "Not only because of the accident, but also because of my weight."

The day after her accident, Perdue said, she connected with a coach from Optavia's Optimal Weight 5 and 1 plan and set out on her weight-loss journey.

"After I had my son [I was] just thinking to myself, ‘I can barely go up and down the stairs to put him in his bed for a nap; how I am going to be able to keep up with him when he's a toddler,’" she recalled. "I committed to lifelong transformation, one healthy habit at a time.

Perdue said she pays it forward by helping coach others in the Optavia program and, thanks to that, she "can finally do all the things I've always wanted to do."

Morgan Root, 34, lost nearly 130 pounds after reaching a peak weight of over 250 pounds.

Root served nearly six years of active military duty but her structured healthy lifestyle came to a halt shortly after she left the military.

Her husband was deployed to Afghanistan during her first pregnancy, Root said, so she turned to food for comfort and gained 80 pounds.

"When my son was six-months-old, I got pregnant with my daughter so the back to back pregnancies is really when I put on the weight," Root explained of her dramatic gain.

The mother of two from Leavenworth, Kansas, realized she was struggling to keep up with her kids, so she turned to Nutrisystem to get back on track.

"I just didn't have the energy to keep up with my kids ... I was missing out on those moments that you should have with a newborn and a toddler," Root said.

She also used social media for motivation and accountability and began working with a personal trainer and learned to love exercise.

"You have to find your reason and your passion," Root said on "GMA." "My kids used to be my excuse for why I couldn’t go to the gym and now they’re my reason for everything I do."

Sara Cloutier, 36, lost around 150 pounds and just completed her first marathon.

"The feeling of crossing the finish line is indescribable," Cloutier said. "I trained so hard. It was just amazing. I did it with one of my friends because accountability is so important."

The mother of two and stepmother of two from Fremont, New Hampshire, said she hit her breaking point when she weighed nearly 300 pounds after the birth of her first daughter.

"I knew that at that weight I wasn't going to be around for her whole life," said Cloutier, who is now pregnant. "I wasn't going to be able to play with her and enjoy things with her because I could barely move at that point."

Cloutier said she got hooked on group fitness classes and now owns her own gym where she teaches group fitness classes daily.

"My family was a huge driving factor for me. They motivated me so much but when I started group fitness classes, I fell in love," she said. "It is so fun."

All three women shared their full stories in People magazine's “Half Their Size” issue, on newsstands Friday.

The magazine chose to feature Root, Perdue and Cloutier because each one made positive changes and lost weight without surgery or gimmicks, People editor Zoe Ruderman said today on "GMA."

"We’re looking for people who made changes that all of us can make so this isn’t about surgery," Ruderman said. "It’s not about gimmicks. This is about real, hard work."

Ruderman credited the women for setting "realistic goals" along to the way as they overcame "really huge obstacles."

"All of these women, they exercised more, they changed how they ate and they set realistic goals," she said. "I think that’s what’s really important."