You can also press the power button and then when the option to power off, restart etc comes up on the screen, long press on the Power Off option and it will ask if you want to reboot to safe mode.
Don't have an Xperia Z3. Android devices have different ways to enter recovery mode, though usually involve some combination keys. Moreover, some third party tools can boot most phones into recovery mode easily. You can check a tutorial here.
My S/ xperia z3 is hanging. I have pressed stop(poers) botton + Volume up for long time. It goes off, bjt comes back to the same Display. I cant RESTART OR TURN OFF. I NEED HELP.