The accident occurred when a stair riser came apart and struck the workers while 50 Cent was in the midst of his performance, according to The Hollywood Reporter.
Two of the stagehands were hit in the head, while another was struck on the elbow.
One of the injured men was heard saying he was "fine" following the incident, but joked that he couldn't believe it occurred "on the f**king last night of the show".
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All three injured stagehands were apparently taken to nearby Cedars-Sinai Medical Centre to be checked by doctors.
Last night's finale saw Melanie Amaro triumph over Josh Krajcik and Chris Rene to win The X Factor's $5m grand prize.
Amaro was overcome with emotion following her win, telling X Factor host Steve Jones and mentor Simon Cowell: "God is good! I'm so happy... This is something I have wanted all my life!"
Justin Bieber, Pitbull, Stevie Wonder and Ne-Yo also performed during the Fox reality series' grand final.
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Watch Melanie Amaro win The X Factor below: