The components draw what they need. Other components do draw from the +3.3V rails but they are not immediately obvious to me.
This is the reason why some companies can label a power supply that can only supply 600W to the +12V rails as a 900W power supply. They up the capability of the +3.3V rails to handle higher current, which it will never draw, to make up for the lower +12V rail capabilities. What's relevant is that the most power hungry components all draw from the +12V rails to a large extent.