An incident-strewn race on a damp track, which was shortened by three laps due to continuing concerns over tyre safety, saw the safety car run for much of its 12 laps.
Collet, who lined up fourth for Van Amersfoort Racing, took third out of Turn 1 before moving into second before the Kemmel Straight.
He then took the lead from Taylor Barnard (Jenzer) at Blanchimont before the first safety car was deployed.
Enduring two restarts, the race then finished under the safety car as dark clouds threatened rainfall.
It was Collet’s first F3 win since Zandvoort last year, with Barnard securing second for his maiden F3 podium ahead of Prema driver Paul Aron.
With the track still damp from Saturday’s heavy rainfall and with more rain set to fall, poleman Hugh Barter (Campos) led off the line.
Barnard took second from Aron at Turn 1 before the Estonian and Collet fought for third, with Collet emerging ahead.
Source: Autosport