In the afternoon session at the Bahrain International Circuit on Wednersday, Russell had a time of 1min 33.459sec, surpassing Piastri's 1:33.739 which had led the way for a while. Leclerc then finished 0.28sec behind the leader.
World champion Lando Norris was half-a-second back in fourth place.
It is the second three-day testing event to be held in Bahrain this year ahead of the opening race of the season in Australia next month.
The teams tested their new cars earlier in the year at the Circuit de Barcelona-Catalunya. Regulation changes have made cars smaller and lighter, with key roles being the charging of an on-board battery and using electrical energy to boost speed.
Max Verstappen, who last week said the new cars are no fun to drive and that the series no longer feels like F1, sat out both sessions.
Red Bull teammate Isack Hadjar finished sixth fastest despite managing just 13 laps in the morning session with a water system issue.
Red Bull team boss Laurent Mekies rejected a suggestion Verstappen could lose interest in Formula One because of the sport's new engine era.
A Dutch reporter told Mekies at a press conference during pre-season testing in Bahrain that the driver had said he had not wanted to drive the 2026 car on the simulator last year because he disliked the feeling so much.
"Have you any concerns of him losing interest in this kind of Formula One?," he was asked.
"Short answer is no," Mekies replied. "Zero concern about that.
"And yes, I do recall our conversation last year when he was switching from a car model to another car model. And yes the difference was so big that at some stage I think rightly he decided to focus on the '25 car.
"The reality is that the challenge of these regulations is massive...but that's also what we love to try to break through these challenges. Try to find solutions that we felt were not on the table. And that's what we will do with Max."
