The board has a power supply input named X1 , the voltage that is connected there goes through a NCP1117 regulator to generate the 5v supply that feeds the 5v parts
and the 5v supply is then fed to another regulator LP2985-33 that outputs the 3.3v in order to supply the 3.3v parts
Apart from that, there is an available input that is intended to supply the board from the a USB cable (5v)
According to the board specs for the supply input X1
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V
The high input voltage limit of 20v is forced by the maximal input voltage of the NCP1117, a higher voltage would damage the regulator.
Basically you can only connect voltages to two points, one is the USBVCC which i intended for regulated 5v supply (from USB port or 5v wall supply) and the other one named X1 where you should connect a supply that is between 6v-20v and which is where you should connect the 8v or 12v (referring always to DC voltages) you talk about.
There is no 3.3v input.