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The Walkinshaw Andretti United driver edged Will Davison by 0.0586s by the end of the 20-minute hit-out at Wanneroo Raceway, with Repco Supercars Championship leader Will Brown third at another 0.0338s off the pace.
Several teams took the opportunity to practice pit stops in the opening minutes of the session, as Triple Eight duo Broc Feeney (#88 Camaro) and Brown set the early pace, the former on a 0:55.5164s and then a 0:55.1286s.
Will Davison (#17 Shell V-Power Mustang) then moved the benchmark to a 0:55.1189s as both of the Red Bull Ampol Camaros returned to pit lane, Brown reportedly with a smoking right-front tyre on Car #87.
Mostert took over the ascendancy with a 0:55.0818s in the #25 Mobil 1 Optus Mustang before Brown, showing no ill effects from whatever drama he may have had minutes earlier, clocked a 0:55.0792s.
Mostert hit back almost immediately with a 0:54.9868s and they were the top two with eight minutes to run in Practice 3.
WAU was also raising eyebrows when it joined rival teams in practicing four-tyre pit stops, despite changing sides only on both #25 and Ryan Wood’s #2 Mobil 1 Truck Assist Mustang in Race 9.
With three minutes to go, Mostert was out of the #25 Mustang as Davison moved back up to second on a 0:55.0454s.
Meanwhile, Wood was on to vault into the top 10 until he was baulked by heavy traffic.
He did find some clear track to set a 0:55.1196s next time around, which was good enough for fifth even though his tyres may have already been compromised.
That was the last of the big moves, meaning a top five of Mostert, Davison, Brown Thomas Randle (#55 Castrol Mustang) on a 0:55.1135s, and the Kiwi rookie.
Feeney ended up sixth with his first-run 0:55.1286s, from Anton De Pasquale (#11 Shell V-Power Mustang), James Courtney (#7 Snowy River Caravans Mustang), Matt Payne (#19 Penrite Mustang), and Cameron Hill (#4 Supaglass Camaro).
Saturday podium finisher Cam Waters (#6 Monster Energy Mustang) ended up 13th, Brodie Kostecki (#1 Erebus Motorsport Camaro) 16th, Richie Stanaway (#26 Penrite Mustang) 17th, and Andre Heimgartner (#8 R&J Batteries Camaro) 18th.
Qualifying for Race 10 of the season starts at 12:00 local time/14:00 AEST.
Results: Practice 3
| Pos | Num | Team/Sponsor | Driver | Car | Fastest lap | Split |
| 1 | 25 | Mobil 1 Optus Racing | Chaz Mostert | Ford Mustang GT | 0:54.9868 | |
| 2 | 17 | Shell V-Power Racing Team | Will Davison | Ford Mustang GT | 0:55.0454 | 0:00.0586 |
| 3 | 87 | Red Bull Ampol Racing | William Brown | Chev Camaro ZL1 | 0:55.0792 | 0:00.0924 |
| 4 | 55 | Monster Castrol Racing | Thomas Randle | Ford Mustang GT | 0:55.1135 | 0:00.1267 |
| 5 | 2 | Mobil 1 Truck Assist Racing | Ryan Wood | Ford Mustang GT | 0:55.1196 | 0:00.1328 |
| 6 | 88 | Red Bull Ampol Racing | Broc Feeney | Chev Camaro ZL1 | 0:55.1286 | 0:00.1418 |
| 7 | 11 | Shell V-Power Racing Team | Anton De Pasquale | Ford Mustang GT | 0:55.1459 | 0:00.1591 |
| 8 | 7 | Snowy River Racing | James Courtney | Ford Mustang GT | 0:55.1935 | 0:00.2067 |
| 9 | 19 | Penrite Racing | Matthew Payne | Ford Mustang GT | 0:55.2263 | 0:00.2395 |
| 10 | 4 | Supaglass Racing | Cameron Hill | Chev Camaro ZL1 | 0:55.2272 | 0:00.2404 |
| 11 | 3 | CoolDrive Racing | Aaron Love | Ford Mustang GT | 0:55.2532 | 0:00.2664 |
| 12 | 200 | PremiAir Nulon Racing | Tim Slade | Chev Camaro ZL1 | 0:55.2639 | 0:00.2771 |
| 13 | 6 | Monster Castrol Racing | Cameron Waters | Ford Mustang GT | 0:55.2655 | 0:00.2787 |
| 14 | 9 | Erebus Motorsport | Jack Le Brocq | Chev Camaro ZL1 | 0:55.3230 | 0:00.3362 |
| 15 | 12 | SCT Motorsport | Jaxon Evans | Chev Camaro ZL1 | 0:55.3260 | 0:00.3392 |
| 16 | 1 | Erebus Motorsport | Brodie Kostecki | Chev Camaro ZL1 | 0:55.3347 | 0:00.3479 |
| 17 | 26 | Penrite Racing | Richie Stanaway | Ford Mustang GT | 0:55.3352 | 0:00.3484 |
| 18 | 8 | R&J Batteries Racing | Andre Heimgartner | Chev Camaro ZL1 | 0:55.3399 | 0:00.3531 |
| 19 | 14 | Middy’s Racing | Bryce Fullwood | Chev Camaro ZL1 | 0:55.3532 | 0:00.3664 |
| 20 | 10 | Bendix Racing | Nick Percat | Chev Camaro ZL1 | 0:55.3782 | 0:00.3914 |
| 21 | 31 | PremiAir Nulon Racing | James Golding | Chev Camaro ZL1 | 0:55.4015 | 0:00.4147 |
| 22 | 20 | TRADIE Beer Racing | David Reynolds | Chev Camaro ZL1 | 0:55.7135 | 0:00.7267 |
| 23 | 96 | Pizza Hut Racing | Macauley Jones | Chev Camaro ZL1 | 0:55.8916 | 0:00.9048 |
| 24 | 18 | DEWALT Racing | Mark Winterbottom | Chev Camaro ZL1 | 0:55.9569 | 0:00.9701 |
