I want to use at the same time 3 stepper motors or maybe 2 stepper motors and 1 servo. The 1 servo or stepper motor will be really huge meaning it will make use of many volts. While the others an average size and make use of 8-12 volt.
I dunno which board to choose because I don’t know which board can support all of them! I don’t want them necessarily turning all together but at least the 2 smaller stepper motors turning at the same time.
Not only because of your voltage needs, but it is better that you use separate power supply lines for the motors and the arduino. This is supported by most stepper drivers out there so you won’t have any special problem connecting those. Servo motors also have independent control and power lines so you can connect them to a power supply other than the arduino’s voltage pins.
Given those I think that an arduino uno and a few stepper drivers will do the job:
Also google “voltage regulator circuit” as with those you can use a single power source and distribute the power to each motor and the Arduino (Give some high capacitance to the input/output of your motors here). Make sure that you conform with the Amperage requirements of all of your components. Furthermore look for Step Up/Down Switching Regulators and voltage boosters, as if you need really high voltage you may need to boost your battery output a bit.