To boot in USB AMSS Download Mode (adanced mode allowing to flash phone's partitions from a PC) press :
CAMERA* + VOLUME UP + POWER (2 vibrations)
To boot in SD Download Mode (adanced mode used to flash updates, generally OTA updates, including system + baseband) press :
CAMERA* + VOLUME DOWN + POWER (3 vibrations)
To boot in Bootloader Mode press :
VOLUME UP + POWER (phone vibrates) then release POWER but keep VOLUME UP pressed.
To boot in Recovery Mode (thanks to Merou09) press :
VOLUME UP + VOLUME DOWN + FOCUS* + POWER (phone vibrates) keep all keys pressed for a few seconds, then release briefly FOCUS* and press it again.
* The key to take photos has two notches, the first one for focus and the second one to take a photo, so for FOCUS press gently and for CAMERA press fully."
Translated from Vache topic on a french forum.