3V/2A is just a way of saying that each coil (phase) will draw 2A when 3V is applied to it - or that the coil resistance is 1.5 ohms. This is with plain DC or sine wave voltage mode drive - so the peak voltage cannot exceed 3V and the drive circuit should be able to source/sink 2A peak.
It also means that we may exceed 3V as long as we do not exceed 2A at any time - stepper motor drivers do this all the time with PWM control of the current - and require presetting the current limit (the "adjustable output current") to this before using with the motor. In other words, stepper drivers operate in current mode.
Thank you for your prompt answer!
But this is not yet clear to me:
option A: should I use the driver TB6600 (powered with a 12V/3A power supply) directrly? In this case I set the driver to 2A, and because the coil resistance is 1,5Ω, the motor input voltage ( = driver output voltage) will be set automatically to 3V (V=I*R)?
option B: should I use the driver TB6600 (set to 2A) (powered with a 12V/3A power supply) AND add a step down converter, between the driver and the motor, to convert from 12V to 3V (3A)?
Option A. You power the TBA66xx with a supply that can provide the TBA66xx recommended voltage (12VDC to 36VDC) and can provide at least 2A (2.5 to 3A preferred just for some additional margin).
This does not mean that the driver will apply the full 36V to the motor - it will ensure whatever voltage is needed to cause proper torque is applied along with chopping or PWM control to make sure that the motor current does not exceed 2A. The voltage across the coil will be discontinuous - PWM or chopped, and not necessarily 3V. But if you monitor the current through the coils it will appear to be somewhat sinusoidal - more or less depending on the microstepping mode selected.
Were you to drive the motor by a more ordinary linear drive such as sine waves then in all probability it will be a voltage drive and the peak voltage from the driver cannot exceed 3V (implying a max. current of 2A for a coil resistance of 1.5 ohms).
You expect too much of motor manufacturers, many of them don't understand current-driven steppers and expect every datasheet to give a voltage for each motor as a matter of course.
You can run a stepper in voltage mode, at a much reduced speed, in fact, often at
a very inconvenient voltage too (less than most motor drivers can work at), but most modern low-impedance steppers are not designed for this. When a stepper is moving fast the back-EMF from the winding inductance and from the motional EMF too are often far higher than the nominal voltage of the winding treated as a resistor - in other words the winding is best thought of as an inductor.