Rhett's post was a shot of himself holding Willa Gray as the pair stood in front of the water, Willa Gray smiling happily at her dad.
"Happy birthday sweet baby girl! How in the world are you 3 already?!?!" Rhett wrote. "I need to pause life cause it's just moving too fast. I love you more than anything in the world."
Akins also shared her own post, gifting fans with a slideshow of sweet photos of Willa Gray.
"My sweet, smart, soooo goofy beautiful baby girl who made me a mama," Akins shared in her caption. "Happy 3rd birthday cutie pie!!!!! Life is so fun with you and you are the BEST big sister to Ada James."
Rhett and Akins adopted Willa Gray from Uganda, bringing her home with them last spring, and Akins' slideshow included multiple snaps of the pair bonding in Willa Gray's home country.
There was also a shot of the 3-year-old celebrating her first Christmas in the States and standing next to a pink Christmas tree.
Along with Willa Gray, Rhett and Akins are also parents to 1-year-old daughter Ada James, and Akins made sure to share an adorable sister snap in her slideshow featuring Willa Gray giving her baby sister a big kiss on the cheek.
"Your sweet little spirit makes the world around you a better place," she continued. "I am so proud of who you are Willa Gray girl. Love you to the moon and back always."
In August, Rhett told PEOPLE that he feels as if Willa Gray has always been a part of his and Akins' family.
"The word 'adopted' doesn't even make sense in my head anymore really," he said. "I know that it happened and I remember it very vividly, but it kind of feels like she has just always been here. Even though Willa Gray comes from [a] whole different part of the world, she's ours and it feels like she never wasn't."
The "Sixteen" singer added that he wants Willa Gray to grow up appreciating her heritage.
"I hope God has given me all the right tools to raise Willa Gray to know that no matter where she comes from, she is our child, and that we want her to express her diversity," he shared. "She's from Tennessee and she's from Uganda and I want her to be proud of that."
Photo Credit: Instagram / @thomasrhettakins