The quartet will join series favorites Ben Whishaw and Hugh Bonneville, who are set to return.
His daughter, Gina Cabot, will be played by Rachel Zegler.
Mortimer joins the Brown family as Mrs. Brown, replacing Sally Hawkins, who portrayed the character inPaddingtonandPaddington 2.
Olivia Colman, Antonio Banderas, Rachel Zegler & Emily Mortimer.Alberto E. Rodriguez/Getty Images for Disney/Diego Radames/Europa Press via Getty Images/Arturo Holmes/Getty Images/Jacopo Raule/Getty Images
Sony Pictures holds rights for the rest of the world, excluding Russia, China, and Japan.
The threequel is being produced by Heyman and Rosie Alison with Rob Silva as co-producer.
The two previous films scored more than $500M between them at the box office and both were BAFTA-nominated.
She will embody the essence of Mary Brown and yet make it utterly her own.
She added: However, I am, and will always be in love with Paddingtons world.
They both gave me so much joy.
I will forever hold them so very close to my heart indeed.
Bonneville is represented by Gordon & French.
Banderas is repped by NuCo Media Group and Hirsch, Wallerstein, Hayum, Matlof, and Fishman.
Zegler is represented by CAA, Narrative, and Freundlich Law.
Mortimer is represented by Tavistock Wood.
