Marciello and Juncadella both went fastest when they were at the wheel of the #88 ASP Mercedes-AMG GT3 in their respective sessions, while Gounon was third quickest in the final 15-minute segment of qualifying for the double-points round of the GT World Challenge Europe.
Their average time of 2m18.433s put them four tenths clear of the Garage 59 Aston Martin Vantage GT3 shared by Nicki Thiim, Marco Sorensen and Ross Gunn, which ended up on 2m18.867s.
Best Audi R8 LMS GT3 was the factory Attempto car of Mattia Drudi, Christopher Mies and Dennis Marschall, which was a further two tenths back on 2m19.090s.
The #4 Haupt Racing Team Mercedes claimed fourth position in the hands of Maro Engel, Luca Stolz and Vincent Abril.
The two Walkenhorst-run factory BMW M6 GT3s took fifth and sixth positions.
Sheldon van der Linde, Marco Wittmann and David Pittard ended up with a 2m19.145s average, which just shaded the 2m19.156s from team-mates Timo Glock, Martin Tomczyk and Thomas Neubauer.
#95 Garage 59 Aston Martin Vantage AMR GT3: Nicki Thiim, Marco Sorensen, Ross Gunn
Photo by: SRO
The CMR Bentley Continental GT3 driven by claimed seventh overall as the top Silver Cup class entry with Stuart White, who was fastest in Q1, Nelson Panciatici, Jean Pierre-Alexandre and Ulysse De Pauw.
The fastest entry from Porsche, winner of the past two editions of the Spa enduro, was the ART-run GPX Racing entry shared by Earl Bamber, Matt Campbell and Mathieu Jaminet in eighth position.
Ricardo Feller set the fastest time across the four mini-sessions with a 2m18.067s in the final 15 minutes aboard the #14 Emil Frey Lamborghini Huracan GT3 Evo.
That moved the Silver Cup entry he shares with Alex Fontana and Rolf Ineichen up to ninth position two tenths up on the sister car driven by Jack Aitken, Arthur Rogier and Konsta Lappalainen.
The top 20 cars from Thursday’s opening qualifying period go through to Friday evening’s Super Pole session.
The factory WRT Audi that headed both sessions of free practice in the hands of Kelvin van der Linde and then Dries Vanthoor failed to make progress to the final session.
Vanthoor failed to set a time in a Q2, which was interrupted by two red flags.
The works Sainteloc Audi of Markus Winkelhock, Patric Niederhauser and Christopher Haase also failed to make the cut.
The 30-minute Super Pole session is scheduled to begin at 7:50pm local time on Friday.