Hitech TGR's Keanu Al Azhari set the Thursday testing pace at Donington Park, with two laps late in session three proving enough to top the times.
Having set a 1m22.521s with five minutes remaining in the final session, the Emirati then lowered the benchmark to a 1m22.379s as the chequered flag flew, putting him just under two tenths ahead of Rodin's championship leader and session two pace-setter Alex Ninovic.
Ninovic's best lap came with a fractionally better time in session two, putting him 0.186s behind at the end of the day. The rest of the top-16 drivers all set their best laps in session three.
With the sun setting, Al Azhari's teammate Deagen Fairclough was third quickest, a tenth of a second further back and 0.027s faster than Xcel's session one pace setter Patrick Heuzenroeder.
VRD Racing's Enzo Tarnvanichkul shot up to fifth quickest at the end of the day, a tenth quicker of Argenti with Prema's Reza Seewooruthun, who completed a top-six covered by just over half a second.
Lucas Fluxa ended his first official day with Hillspeed with the seventh fastest lap, 0.074s ahead of Rodin's Gianmarco Pradel, with JHR duo Noah Lisle and Kai Daryanani completing the top-10, which was split by 0.720s.
The top-14 drivers were within a second of the fastest lap, with RACE LAB's Max Taylor the quickest newcomer in that position, with JHR's Bart Harrison 16th, Fortec's Liam McNeilly 17th and Xcel's Kabir Anurag 19th.
Three more sessions follow on Friday ahead of the penultimate rounds of the season.
GB3 Championship, partnered by the BRDC, Donington Park Thursday testing combined result:
1. Keanu Al Azhari, Hitech TGR, 1m22.379s
2. Alex Ninovic, Rodin Motorsport, +0.186s
3. Deagen Fairclough, Hitech TGR, +0.301s
4. Patrick Heuzenroeder, Xcel Motorsport, +0.328s
5. Enzo Tarnvanichkul, VRD Racing, +0.407s
6. Reza Seewooruthun, Argenti with Prema, +0.522s
7. Lucas Fluxa, Hillspeed, +0.549s
8. Gianmarco Pradel, Rodin Motorsport, +0.623s
9. Noah Lisle, JHR Developments, +0.706s
10. Kai Daryanani, JHR Developments, +0.720s
11. Abbi Pulling, Rodin Motorsport, +0.764s
12. Jack Sherwood, Xcel Motorsport, +0.782s
13. Flynn Jackes, Elite Motorsport, +0.894s
14. Max Taylor, RACE LAB, +0.912s
15. Patricio Gonzalez, VRD Racing, +1.315s
16. Bart Harrison, JHR Developments, +1.385s
17. Liam McNeilly, Fortec Motorsports, +1.410s
18. Rodrigo Gonzalez, VRD Racing, +1.437s
19. Kabir Anurag, Xcel Motorsport, +1.523s
20. Bianca Bustamante, Elite Motorsport, +2.637s