The Bluebirds are currently 18th in the Premier League, two points adrift of safety, but Marshall believes that six points from their remaining three games could see them climb out of the bottom three.
"We're on 30 points at the moment so we can definitely get to 36, which could be enough," the 29-year-old told members of the media.
"We've got two big games against Sunderland and Newcastle and then Chelsea, which is going to be difficult at the end of the season.
"But we've shown a lot of battle and quality in recent performances to show we're more than capable of doing that."
Cardiff travel to the Stadium of Light to face Sunderland on Sunday.