Carrie Underwood, Gabby Barrett and Thomas Rhett are among the star-studded bill of performers helping celebrate the Season 20 finale of American Idol.

According to People, the three country superstars are just some of the all-genre roster of performers helping to celebrate the season's three-hour grand finale episode Sunday night.

Underwood and Barrett each have a special connection to Idol: Underwood won the televised singing competition in 2005, and she returned to the Idol stage on Sunday night (May 15) as a guest mentor. Barrett finished third on Season 16 of the show, and has since gone on to achieve crossover country superstardom with the success of songs like "I Hope" and "The Good Ones."

Rhett's Idol finale performance will be a duet with show judge Katy Perry. The pair recently collaborated on a duet called "Where We Started," which is the title track of an album Rhett put out in April.

All three of the judges are also performing during the finale, meaning that Luke Bryan and newly-minted Rock & Roll Hall of Fame inductee Lionel Richie will take the stage, too. '90s country mainstay Deana Carter is also on the lineup, as are several more stars from a wide array of genres. Ben Platt, Flo Rida, Melissa Etheridge, Tai Verdes, James Arthur and Earth, Wind & Fire are also performing.

Additionally, Michael Bublé will perform with Christian Guardino, a contestant who made it into the final seven on Idol.

During Sunday night's semi-finals, the top five American Idol contestants were whittled down to a final three: HunterGirl, Noah Thompson and Leah Marlene. Those finalists will vie for the title of Idol champ — and a grand prize which includes roughly $250,000 and a recording contract — during the finale episode.

Country 'American Idol' Stars Who Disappeared

We cheered for them. We voted for them. We followed them. Then, these seven American Idol stars vanished. Or, at least it seems like it.

What happened to some of country music's best American Idol contestants over the last 20 years? This list of country American Idol stars who disappeared includes a winner, a runner-up and a third place finisher, plus several early cuts who enjoyed radio success before losing touch with fans.

PICTURES: See Inside Carrie Underwood's Beautiful Post-'American Idol' Home

Carrie Underwood invested her money wisely after winning American Idol in 2005. One of her first big purchases was a 3,099-square-foot home in the well-to-do Nashville suburb of Franklin, Tenn. Underwood paid $384,000 for a luxurious 3-bedroom, 3.5-bathroom stone and stucco home in a private gated community.

The house features a library, a vaulted ceiling and lovely amenities throughout, but while it's very nice, it's certainly not the kind of home you'd picture for a budding country superstar. Underwood wound up being a huge success right out of the gate with her debut album, Some Hearts, and she sold the home in Franklin for $372,500 in 2007, when she purchased the Brentwood mansion that she would later share with her husband, Mike Fisher.