Scotland beat Argentina 29-6 to win the second test after losing the first 26-18 at San Salvador de Jujuy.
Injury to full back Rory Hutchinson (26) hands Ollie Smith (21) his debut in one of four backline changes for the match in Santiago del Estero.
Rufus McLean (22) comes in for Darcy Graham (25) on the wing while Sione Tuipulotu (25) is selected in place of Sam Johnson (29) in the midfield. Ali Price (29) is in at scrum half despite Ben White’s (24) stellar showing in last week’s second test victory in Salta that tied the series.
In the front-row, Zander Fagerson (26) will make his 50th appearance and is joined by Sale Sharks’ hooker Ewan Ashman (22), who will make his first start in a test, and Rory Sutherland (28) returns to the team after missing last week’s second test.
In the second row Jonny Gray (28) replaces Grant Gilchrist (31) who captained the side in the first two tests.
Watson will captain the team for the first time.
Lock Glen Young (27) is in line to earn his first cap after being included on the bench for the first time.
Meanwhile, Argentina have reacted to last weekend’s defeat to Scotland by making 11 changes to their team.
The home side have lost seven players to injury with coach Michael Cheika replacing Julian Montoya (28) as captain with Pablo Matera (28), who returned to the role he held for two years between 2018-2020.
Montoya suffered a back injury last week while Matera sat out the second test after being hit in the cheek bone in the first test at San Salvador de Jujuy, which Argentina won 26-18.
Seven of the forwards from last Saturday are changed with only lock Guido Petti (27) keeping his place.
Veteran hooker Agustin Creevy (37) will start his first test since the 2019 World Cup.
Lautaro Bazan Velez (26), who only three months ago was part of Argentina’s sevens squad, will debut at scrum half with similarly uncapped hooker Ignacio Ruiz (21) and fly half Tomas Albornoz (24) on the bench.
Juan Cruz Mallia (25) is back at full back with Emiliano Boffelli (27)moving to the wing and Matias Moroni (31) back at centre for the injured Jeronimo de la Fuente (31).
