Nico Hulkenberg goes quickest, ahead of Mercedes and Ferrari
Red flags: Norris in the morning session in the gravel, Hulkenberg, and Giovinazzi stop on track
Ferraris quickest on first two days; Toro Rosso's Kvyat quickest on day three
Williams two days late to Barcelona
Since the teams are all running different secret programmes during testing - some of which have changed daily - the tyre or fuel level information isn't available to the media alongside lap times.
But in an attempt to clear things up on which tyres are which, Andrew Benson has more.
F1 Debate: If Leclerc could realise the same 0.4 improvement with the C4s that Lewis did, that would give him a 1m17.665s, fastest of the week.
Ronny Welter: Say what you want about @kvyatofficial but after everything that he's been through, he's still in F1 racing for a solid team. Both Helmut Marko and Daniil deserve praise for that: Daniil for not giving up and Helmut for giving him yet another shot.
Cameron F1: Wow, well who expected Daniel Ricciardo to go fastest there? I certainly didn't but an awesome performance from him, Renault's first podium in the making maybe...
Lord Roger: I didn’t think that Pirelli could make the tyres any more confusing but they’ve only gone and proved me wrong. C1 is hardest and C5 is softest? Or vice versa? Bring back the conventional names. Even if there was 20 of them, it’d be more straightforward
Chris Medland
F1 journalist in Barcelona
Alfa Romeo is one of the teams that has really caught the eye so far this week. Aside from Kimi Raikkonen's impressive lap time on Wednesday, the C38 has run reliably so far and has also seen a number of updates added already.
There's a new engine cover already on the car, as well as a T-wing at the rear, both of which were not present on day one. It's a far cry from where the team was as Sauber a few years ago when it had such financial worries. Now it has the funds to run a special livery during its shakedown, bring significant new parts during testing and employ a drivers' world champion in Kimi Raikkonen.
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Dale Foster: Not much to judge from but I will stick my neck out and say Lando Norris and George Russell will fair well, Alex Albon will struggle at first and Helmut will get the driver guillotine sharpened!! #bbcf1 #F12019 #f1testing
Image source, Getty ImagesHe beats Lewis Hamilton's time by six-hundredths of a second.
He's on the C4 tyres.
Image source, Getty ImagesThere's really not a lot to judge from after only three-and-a-bit days of testing, but everyone has first impressions...
Alexander Albon is driving for Toro Rosso, Lando Norris for McLaren, and George Russell is in the Williams and has had even less track time.
What do you think?
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Euan: And Mercedes are back to the top. Did anyone really think that they wouldn't be?
Martin Roberts: The Mercedes is allowed to stretch its legs now.
F1W11YT: How is Kubica still only on 12 laps?? Come on Williams stop faffing and get the car out on track, your driver need the experience and you need the data!!
Is this going to be a straight swap all day between Ferrari and Mercedes?
Meanwhile, Lance Stroll has had his first spin at the chicane at Turn 13. He does a 180 to face the wrong way, but he continues on.
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Now that you've all had chance to see the 2019 cars and liveries, you can vote for your favourite here.
If you want another look, here's the gallery.
Sadly, you can't vote for this lovely Alfa Romeo as it was a one-off livery.
Image source, Alfa Romeo TwitterThe new times are:
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Andrew Benson
BBC Sport’s chief F1 writer
That's more like it from Hamilton and Mercedes. An 18.8 on the C2 tyre, a sniff ahead of Leclerc's Ferrari, which equates to 18.1/18.2 on the C3, just on tyre correction. A chunk better than anything they had achieved until today.
Hamilton goes quickest with a 1:18.800.
Image source, Getty ImagesPhil: Mance: I know I'm going to pay for this but... the McLarens are looking promising!
Bob McFeil: I have a bad feeling McLaren are going to have a bad day. Someone tell me it will go fine...
Kumaran Ravendradas: Interesting to hear @andrewbensonf1, external views on Mercedes. I just wonder if they are working harder to keep things quiet. The number of laps they have completed have been right at the top if not the top!
Carl Slocombe: @McLarenF1 Looking solid if unspectacular, which is fantastic news given how they've been for the last 4 years. Hope that the lack of development on last year's car has meant they put in a double shift on this car. Nipping at Red Bull this year given the engine rumours?
Image source, Pirelli TwitterCome on then, tweet us with the #bbcf1 hashtag.
Who's hot and who's not this week?