Rosetta joins her older brothers Lenny, born in 2018, and Archie, born in 2016.
Bec and George announced they were expecting earlier this year, in a post shared to Instagram.
“When we said it was going to be a huge year, we meant it!” Bec wrote.
“Baby #3 is on its way. Our lives are about to get a whole new level of crazy and we can’t bloody wait.”
Little Rosetta’s name appears to be a touching tribute to Bec’s late mother Rosa, who died of breast cancer in 2013 – just weeks into the filming of Bec and George’ season of The Block.
In 2018, Bec told 9Honey the loss of her mother is something that affected the way she felt about being a mum.
"I think it's very difficult to lose a parent and then become a parent yourself," she told outlet.
"You realise how hard it is to be a parent, and you just want to thank them."