Valtteri Bottas set fastest ahead of Daniel Ricciardo and Nico Rosberg; Lewis Hamilton in fifth; Final half hour of session on slicks after morning shower.
The Finn posted a best time of 1:49.465 to finish two-tenths of a second clear of Daniel Ricciardo at the top.
Four-times Spa winner Kimi Raikkonen and Lewis Hamilton finished with equal 1:49.817 times, although fourth place went to the Ferrari due to the fact he set his lap first, with Fernando Alonso in sixth in the second F14 T.
Toro Rosso rookie Daniil Kvyat impressed again to take seventh ahead of former Spa winners Jenson Button (McLaren) and Felipe Massa (Williams).
Third free practice began in wet conditions following rain showers on Saturday morning. All of the drivers completed their installation laps on intermediate compound tires, before sitting in the pits and waiting for the track to dry.
Only Andre Lotterer, Jules Bianchi and Esteban Gutierrez opted to post a time in the first half of the session.