The average current consumed by the stepper motors is what is most important for sizing the power supply. The peak current listed in the datasheet is only achieved for a brief period during the motor coil charge cycle. Not only that, the charge on one motor coil will recirculate and end up charging the other coil. Back EMF is also captured in the driver's capacitors which also ends up charging the motor coils.
Rule of thumb:
Power_supply_current = number_of_motors * 1/3 * motor_peak_current
for typical motors wired bipolar series. However, if you have an 8 wire motor and have wired it bipolar parallel then you will need to double the value calculated above. There is no need to go above this value. The estimate already contains a healthy margin of error.
In testing, I have found that stepper motors often draw an average current about 1/4-1/5 of the rated peak current when idle or running slowly. This goes up at midrange speeds and then drops off at higher speeds. The current goes up in the middle, not because the motor is getting more power, but because of energy losses. It goes down at high speeds as increasing back EMF cancels out the incoming voltage from the power supply.
So finally, to answer the original question. You should only need about 0.33A to run all three motors, assuming they are wired bipolar parallel and your driver does not have any gross inefficiencies.