The news comes as Wainwright is honoured with an OBE in the Queen’s Birthday Honours for services to writing and television.
She said: “I’m writing Happy Valley 3 now, that is the next thing on the list.
“I’ve only just started, I’m getting all the research together but we have got some good storylines in mind."
Happy Valley follows Sergeant Catherine Cawood, played by Sarah Lancashire, as she leads a team of strong-willed police officers in Calderdale to solve cases in the area.
There have been two series of the BBC drama, the last of which aired back in 2016.
The show has won countless awards including winning the Bafta for Best Drama Series twice.
To be able to have the time to write a third series of Happy Valley, Wainwright won't be directing any episodes of Gentleman Jack, TV drama that follows the life of Halifax diarist Anne Lister, which is due to start filming this month.
She added: “I’m not directing Gentleman Jack this time which I’m very sad about but I made that choice because I wanted to get on with Happy Valley 3 and I hope I will be directing that but we will see, it’s just very time consuming.
“All the time I’m directing I’m not writing so it’s a big decision.”