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The Canadian led every lap of the lead except the very first one, having fallen behind Cassidy off the line.
However, he quickly recovered to retake the lead on Lap 2 and then slowly pulled away.
Although Cassidy slightly managed to close the gap towards the end, he had to settle for second, as the podium was rounded out by Hitech's George Russell.
The Briton passed Niko Kari at the start, the Finn making contact with fellow Red Bull junior Sergio Sette Camara, which saw the Brazilian drop to last.
Russell's Hitech teammate Ben Barnicoat ended up as top rookie in fourth, with Maximilian Gunther closely following him, the German recovering from eighth.
Motopark duo Joel Eriksson and Kari took sixth and eighth respectively, sandwiching the fourth Prema car of Ralf Aron.
Harrison Newey and Callum Ilott both made a couple of moves during the race to make up the top 10, the latter passing Pedro Piquet during the dying stages.
|1||Lance Stroll||Prema Powerteam||-|
|2||Nick Cassidy||Prema Powerteam||0.769|
|5||Maximilian Gunther||Prema Powerteam||9.202|
|7||Ralf Aron||Prema Powerteam||13.453|
|9||Harrison Newey||Van Amersfoort Racing||19.159|
|10||Callum Ilott||Van Amersfoort Racing||19.718|
|11||Pedro Piquet||Van Amersfoort Racing||20.747|
|13||Mikkel Jensen||kfzteile24 Mucke Motorsport||24.008|
|14||Guan Yu Zhou||Motopark||24.465|
|16||Sergio Sette Camara||Motopark||34.218|
|17||Anthoine Hubert||Van Amersfoort Racing||34.428|
|20||Zhi Cong Li||Carlin||53.879|
|Ret||Arjun Maini||ThreeBond with T-Sport|
Paul Ricard F3: Gunther completes Prema hat-trick with double pole
Paul Ricard F3: Ilott claims career-first victory in Race 2