Thank for your reply.
My "broken" works few days with the same monitor and the same cable.
After a day of work, I go home and I turn on the Pi without image. I plug into the TV : ok.
Few day after, a friend come with his Pi3, use my SD, cable and monitor with success.
My config.txt was hdmi_force_hotplug=1 and hdmi_boost=0 from the first day.
When it stop working, I try to increase boost little by little, until 15 : no signal for monitors (I tryed differents monitors at my work).
I've contacted GearBest after sales, waiting for an answer (or a new Pi, I hope !).