Behind the two Mercedes, Lando Norris was third fastest though in reality little can be read into the times as conditions changed throughout the hour.
The opening 30 minutes saw the track gradually improve before the weather closed back in and they deteriorated once more.
Rain throughout the morning had left the circuit wet, though eased such that cars did head out as Free Practice 3 began.
Earlier, the Formula 3 Sprint had been postponed due to the conditions, though the standing water that triggered that decision had disappeared.
However, the weather was not expected to ease, prompting drivers out on track to get a read on the low grip circuit on intermediate rubber.
Max Verstappen had a spin at Brooklands shortly after exiting the pit lane, such was the lack of grip on the circuit.
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Just seven minutes into the session, the red flag was shown after Pierre Gasly stopped at the Vale chicane after losing the rear end under braking.
The Alpine driver had beached his car in the gravel trap on the outside of the left-hander and in need of rescue – his practice session over.
Yuki Tsunoda was the fastest at that point, his best a 1:47.080s, more than 20 seconds slower than what Norris had logged in Free Practice 2 on Friday afternoon.
His RB team-mate Daniel Ricciardo caught the eye of officials when he was seen weaving down the pit lane, which stewards confirmed they would investigate after the session.
The red flag lasted six minutes with most heading out on used intermediate rubber.
Initial reports were track conditions had improved under the stoppage, reflected by the fastest lap rapidly dropping; Hamilton lowering it to 1:41.905s – more than five seconds faster than Tsunoda’s early effort.
It remained slippery, Valtteri Bottas spinning at The Loop with 20 minutes of the session having elapsed.
Hamilton was the first to log a lap under 100 seconds, managing a 1:39.546s with 38 minutes remaining.
Small sections of the track were beginning to improve such that dry line was emerging, specifically through Brooklands, Luffield, and Woodcote.
The bulk remained damp at best but highlighted the moving target conditions were offering, resulting in the timesheet changing rapidly as drivers gained confidence and tyre temperature.
The progression made lap times largely irrelevant as a driver’s position on the totem directly related to when they set their time in the ever-improving conditions.
Rain returned with 20 minutes remaining, coinciding with Verstappen banking a lap that left him fifth fastest – a 1:38.393s lap.
Drivers continued to circulate, though Russell’s 1:37.529s was unchallenged at the top of the timesheets, with Hamilton second best, 0.035s slower.
Ultimately, it was a session that meant very little in terms of its outcome, much of which was determined in the opening half hour.
The real benefit was for the drivers ahead of a potentially wet qualifying session later in the day, and a potentially wet race.
It also afforded an opportunity to complete a practice start on the wet track, but otherwise there was little to be gleaned from the session.
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