On Sunday, the former Playmate, who was Hefner's third wife, shared a loving tribute to him on Instagram, thanking him for their time together.
"Three years. It's hard to believe," she said. "Thank you for always being my greatest ally, teaching me the definition of kindness, and making me feel extremely important and adored for ten years. I will be forever grateful."
Crystal, 34, also called him a "legend" in her Instagram Stories.
In 2017, Hefner, 91, "peacefully passed away today from natural causes at his home, The Playboy Mansion, surrounded by loved ones," a rep for the Playboy Enterprises founder told PEOPLE at the time.
Also on Sunday, Hefner's son Cooper, 29, paid tribute on Instagram with a throwback photo that was taken at the Playboy Mansion. "We lost our father three years ago today. This will be an afternoon accompanied by Jack and Pepsi to celebrate him and our continued love. We miss and love you, Dad," he wrote.
Crystal made her debut in Playboy at age 24 as the magazine's December 2009 Playmate. She and Hefner — who had a 60-year age difference — first got engaged in 2010, though she called it off four days before their planned June 2011 wedding.
In the years after his wedding to Crystal, he retreated from the spotlight, settling into a quiet life at home. "Crystal was his constant companion," a Playboy insider previously told PEOPLE. "She was extremely devoted to him."
Crystal recently revealed her first public relationship since Hefner's death. This summer, she said she found love again with her new boyfriend Nathan Levi.
The couple was first linked in July after they were spotted strolling on the beach together and kissing at the Casa de Mita resort in Mexico. She soon made the relationship Instagram official, sharing a photo of the two with the caption: "You feel like home."
And in August, they celebrated Levi's 32nd birthday with a trip to the Four Seasons hotel in Punta Mita, Mexico. "Happy birthday to my favorite person," Crystal said. "Glad we could get away together for your special day."