Prenger was the first to make it through, having topped the public vote with a routine to the 1986 Cameo hit 'Word Up', in which the actress dressed as the funk group's frontman Larry Blackmon.
Judges Arlene Phillips, Greg James and Lee Mack then had to choose between Vanessa Feltz, who danced to Cher's 'If I Could Turn Back Time' in leotard and fishnet tights, and Nelson, who was joined by Britain's Got Talent stars Twist & Pulse for a hip-hop spin on Tchaikovsky's 'Swan Lake'. The trio eventually decided to save Nelson.
Newsreaders Ore Oduba and Sonali Shah and ventriloquist Nina Conti received the fewest public votes and were automatically eliminated.
Oduba and Shah performed to the Outkast track 'Hey Ya', while Conti teamed up with her puppet Monk for a gorilla-themed routine to Reel 2 Reel's 'I Like to Move It'.
The show also featured appearances from Nicole Scherzinger, who sang new single 'Boomerang', and Justin Bieber, who performed an acoustic version of 'All Around the World'.
Prenger and Nelson will join Tim Vine, The Soap Stars, Antony Cotton and Katherine Ryan in next week's final, which takes place on March 9 at 7pm on BBC1.
Let's Dance Show 3