Taking to Instagram, the singer shared a photo taken on their wedding day, writing: "3 years with this guy, 2nd best decision I ever made. 1st was that dress."
Allen and Harbour were first seen together in October 2019, before confirming they were in a relationship the same month. The couple later tied the knot in September 2020 at Graceland Wedding Chapel in Las Vegas, officiated by an Elvis impersonator.
Earlier this year Allen opened up on how she and Harbour first met, revealing they had connected on exclusive dating app Raya.
"It was the first time I'd been on this dating app. I was scrolling through and landed on David's profile and pressed accept," she said during an appearance on on The Jonathan Ross Show.
"I didn't know who he was. I thought he was just like a sexy policeman from a reality TV show because he was wearing a policeman's uniform. It was a still from Stranger Things. I'd never seen Stranger Things."
She added: "When I met him I didn't think it was going to go anywhere... He was only going to be here for a couple of months. And then it did [go somewhere]."
Harbour has also spoken publicly about their relationship, revealing that he has "never met someone who is so deeply kind" as Allen.
"I married a beautiful, incredible woman who I love," he told PEOPLE. "I've never felt so taken care of and cared for by another human being. She really is just a deeply kind individual and I love that about her."
Stranger Things seasons 1-4 are now streaming worldwide on Netflix.
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