Liverpudlian Marcus, who finished second with Gary Barlow on last year's series, said the four piece were the dark horses of the competition.
"These guys are the dark horses and the ones to watch," the Mercy singer said this week. " They've got an amazing work ethic and are so drive. They want it to much."
Marcus revealed: "I know Jaymi from auditioning before and I remember being surprised he didn't go through, so I'm glad he's got his chance. They are so cool and the girls will love them."
However Marcus wasn't as keen on GMD3, who have since renamed themselves to District3.
He told Star magazine about the trio: "They are good but they need to be careful not to let it get dated. "
He advised: "They need to be unique an d not just sing in a three part harmony because the groups are strong this year, they need to stand out."
The X Factor 2012 continues this Saturday night on ITV1, with the results show following on Sunday night with performances from Taylor Swift and Rebecca Ferguson.