The inclusions are set to come at the expense of Damien Cook and Hudson Young, both of whom have paid the price for the hefty loss in Origin II and will not feature in the series finale on July 12 in Sydney.
Wests Tigers prop Stefano Utoikamanu, afforded only 12 minutes of game-time on debut, is also expected to be dropped.
The NRL's leader in try assists, South Sydney five-eighth Walker will not dislodge Jarome Luai from the starting side, with the Penrith playmaker again partnering Mitch Moses in the halves.
Walker has four games of Origin experience, most recently featuring in all three matches of 2020's post-season series.
Fittler deployed him as a bench utility in the first game of that series, a tight loss to Queensland, and will hope the Rabbitohs veteran can solve the bench woes that have haunted NSW in 2023.
The Blues named Nicho Hynes as a utility for Origin I but only played him for 12 minutes late on, and then deployed veteran hooker Cook as a centre in the second game after injury struck down Tom Trbojevic.
Joining Walker in the side for his maiden Blues appearance is Souths teammate Koloamatangi.
An ankle injury scuppered the back-rower's chances of earning a debut in the series opener but he was selected as 19th man for Origin II.
James Tedesco will retain his spot as fullback and captain despite concerns over his form and stiff challenges from Dylan Edwards and Scott Drinkwater in recent weeks.
Prop Payne Haas, NSW's best player in the second game of the series, will be named despite aggravating an ankle injury playing for Brisbane at the weekend.
The Broncos are expected to release an update on his condition on Monday.
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