The 55-year-old was hired after the original script was allegedly rejected by the cast.
Nanny McPhee writer and star Emma will re-write the first draft, put together by Bridget Jones creator Helen Fielding and One Day actor David Nicholls.
A source told The Sun: "Trying to get a third film into production has been a nightmare.
"The cast didn't like the script and they've now got Emma involved in the hope she can add her British charm."
Renee Zellweger, who starred as Bridget in the first two films, is thought to be returning, although Helen previously admitted to losing touch with her.
SHOCK FOR BRIDGET JONES FANS AS NEW BOOK REVEALS MARK DARCEY IS DEAD
Someone who will definitely not be taking part in the new instalment though, is Hugh Grant, who played Daniel Cleaver.
He told Midlands Free Radio: "I decided not to do it. But I think they're going to go ahead and do it without Daniel.
"The book's excellent, by the way, but the script is completely different – well, the script as I last saw it a few years ago."
Helen's latest novel in the series, Mad About The Boy, shows Bridget as a widower in her 50s, raising two children alone after the death of Mark Darcey.
It is believed that the third film will be significantly different, and instead will focus on Bridget in her 40s, desperate to have children and with Mark still alive.
Are you looking forward to the third Bridget Jones film?