The couple have two children - Mia, six, and Lena, two.
Filled with joy, the 42-year-old said: 'It's been a good week for me, had a little scan last week - third Tindall on its way'.
The former England Rugby star, 41, spoke about the good news on the podcast he co-hosts, The Good, The Bad & The Rugby.
But undoubtedly, both are devoted parents to Mia and Lena.
Vote Counting Underway in Ghana’s National Election
They wonder how the Ghanaian electorate could vote so much for the NDC and their presidential-candidate John Dramani Mahama. EU's chief observer Javier Nart said that in Ghana "it's not the Kalashnikov that commands, it is the ballot box".
And the couple know Mia will be over the moon with the news, as she's been desperate for another younger sibling for some time.
The Queen and Duke of Edinburgh said: "Her Majesty and the Duke of Edinburgh are aware and delighted".
Talking about her past experiences, her husband added: "Z is very good, always careful because of things that have happened in the past, and really looking forward to it".
"We're not sure what to do Covi or Covina - I don't know where to go with names".
He said: "It was a masterstroke of the Princess Royal when she decided not to give her children titles". The Duchess of Sussex lost her second child while holding her son Archie in one July morning this year.