The team shows a number of changes after injuries took their toll in the second Test defeat last weekend, with Ugo Monye, Riki Flutey, Shane Williams, Martyn Williams, Joe Worsley, Phil Vickery and Andrew Sheridan all called into the starting XV this week and Ellis one of only two backs named among the replacements.
Croft started both previous Tests, while scrum-half Ellis has been among the replacements but is yet to get a taste of Test action on the tour.
Head coach Ian McGeechan said: "The 2009 Lions have thrown themselves into the spirit of the tour, have interacted with the people of South Africa and shown themselves to be great ambassadors for northern hemisphere rugby.
"They have met many challenges on the tour, including playing two matches per week, the inevitable injuries and two narrow Test losses. Despite this they have remained a tight, focused squad and I know those playing in the Third Test will give their all to ensure that we achieve a victory on Saturday.
"Injuries have had an impact on the tour, as you would expect, including selection for the Third Test as we were unfortunate to lose Brian O'Driscoll, Jamie Roberts, Gethin Jenkins and Adam Jones last Saturday. However, the players coming into the starting XV are all talented international players and are as keen as ever to turn the tables on the Springboks. Last Saturday we were minutes away from winning and that is a huge positive to take into the Third Test.”
Sheridan, Worsley and Martyn Williams will be part of a Lions starting XV for the first time, while Stephen Jones, Mike Phillips, Paul O'Connell and Jamie Heaslip will earn the distinction of being in the starting XV for all three Test matches.
South Africa have made 10 changes to their line-up, with only scrum-half Fourie du Preez, flanker Juan Smith, lock Victor Matfield and props Tendai Mtawarira and John Smit retained from the second Test line-up.
British & Irish Lions (v South Africa, 2pm Saturday)
15 Rob Kearney (Leinster/Ireland)
14 Ugo Monye (Harlequins/England)
13 Tommy Bowe (Ospreys/Ireland)
12 Riki Flutey (London Wasps/England)
11 Shane Williams (Ospreys/Wales)
10 Stephen Jones (Scarlets/Wales)
9 Mike Phillips (Ospreys/Wales)
1 Andrew Sheridan (Sale Sharks/England)
2 Matthew Rees (Scarlets/Wales)
3 Phil Vickery (London Wasps/England)
4 Simon Shaw (London Wasps/England)
5 Paul O'Connell (Munster/Ireland) Captain
6 Joe Worsley (London Wasps/England)
7 Martyn Williams (Cardiff Blues/Wales)
8 Jamie Heaslip (Leinster/Ireland)
16 Ross Ford (Edinburgh/Scotland)
17 John Hayes (Munster/Ireland)
18 Alun-Wyn Jones (Ospreys/Wales)
19 David Wallace (Munster/Ireland)
20 Tom Croft (Leicester Tigers/England)
21 Harry Ellis (Leicester Tigers/England)
22 James Hook (Ospreys/Wales)
South Africa: Kirchner (Bulls); Ndungane (Sharks), Fourie (Lions), Olivier (Bulls), Nokwe (Cheetahs); M Steyn (Bulls), Du Preez (Bulls); Mtawarira (Sharks), Ralepelle (Bulls), Smit (Sharks), Muller (Sharks), Matfield (Bulls), Brussow (Cheetahs), Smith (Cheetahs), Kankowski (Sharks). Replacements: B du Plessis (Sharks), Steenkamp (Bulls), Carstens (Sharks), Sykes (Sharks), Spies (Bulls), R Pienaar (Sharks), F Steyn (Sharks).