Filmed in both Welsh and English, it will air in October 2020 on Welsh-language channel S4C.
The English-language version will then be shown on BBC Wales from February 2021 and across the BBC on iPlayer.
Myles will return as Faith Howells, while Olivier award-winning actress Celia Imrie will join the cast.
Bradley Freegard will also be back as Evan Howells, as well as Mark Lewis Jones as Steve Baldini.
Filming is set to begin on 27 January in Laugharne, Carmarthenshire.
The series is called Un Bore Mercher on S4C and Amanda Rees, the channel’s director of content, said she was “thrilled” to see it returning.
“This pioneering drama is hailed as one of S4C’s originals, and the award-winning thriller, set in the heart of one of Wales’ most beautiful landscapes, has raised the profile of Welsh drama across the world,” she added.
Nick Andrews, head of commissioning at BBC Wales, added: “Keeping Faith is a fantastic show and I’m delighted that it will return for a third series. “
Series one and two of Keeping Faith were massively successful for the BBC and S4C, with more than 33 million iPlayer requests to date.
The show won three Bafta Cymru awards in 2018, with Myles winning best actress for her role as Faith.