The pair have been called up in a 13-man squad to face the Proteas at the Kia Oval following the 340-run defeat at Trent Bridge but there is a familiar feeling of disappointment for Surrey's Mark Stoneman, with misfiring opener Keaton Jennings retained.
The selectors have taken the unusual step of confirming 28-year-old Westley's place in the side, and batting order, after Ballance was ruled out with a fractured finger, but Malan has no such promise and will go head to head with spin-bowling all-rounder Liam Dawson.
If head coach Trevor Bayliss and captain Joe Root decide a second slow-bowling option is necessary alongside Moeen Ali, Dawson will continue his unexpected stint in the team, with Malan on hand as an alternative option to bolster the batting.
Malan's county team-mate Toby Roland-Jones, a third uncapped squad member, will challenge Mark Wood for his place in the side, the latter having struggled for form in the series but been passed fit after a heel complaint.
Westley's call-up has several things to recommend it, not least his right-handedness, with Alastair Cook, Jennings and Ballance all left-handers. His long-time county association with Cook can also do him no harm and it is just two months since the pair scored twin hundreds in a 243-run stand against Hampshire at Chelmsford.
Form-wise, Westley averages 53.11 in the Specsavers County Championship, with two centuries, and just as importantly made 106 not out against the touring Proteas attack for England Lions at Worcester.
An England and Wales Cricket Board statement read: "Essex's Tom Westley is set to earn his first cap, with selectors confirming he will bat at number three in the 100th Test at the Oval.
"Middlesex batsman Dawid Malan is also included in the squad for the first time and could feature if England decide on selecting an extra batsman when they confirm their starting line-up next week.
"England will report to the Kia Oval on Tuesday, where they will train in the afternoon."
Malan was thought to be close to selection at the start of the series, having made a strong impression on the England management with his 78 on his Twenty20 debut against the South Africans last month.
It is thought Root's preference for Yorkshire team-mate Ballance delayed his arrival on the Test scene but he now has a real chance of staking his claim, with Dawson a solid but unremarkable addition to the side at Lord's and Trent Bridge.
England are 6/5 with Sky Bet to win the third Test with the tourists 15/8 and the draw a 3/1 chance.
England squad: Joe Root (Yorkshire - captain), Moeen Ali (Worcestershire), James Anderson (Lancashire), Jonathan Bairstow (Yorkshire), Stuart Broad (Nottinghamshire), Alastair Cook (Essex), Liam Dawson (Hampshire), Keaton Jennings (Durham), Dawid Malan (Middlesex), Toby Roland-Jones (Middlesex), Ben Stokes (Durham), Tom Westley (Essex), Mark Wood (Durham)
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