If the stepper motors you have mentioned are wired bipolar then for bipolar parallel you will need 2 times the name plate rating.
For PSU current rating you need at least 2/3rds the total of all 3 motors.
Do you need stepper motors with such high holding torque ratings?
Single and dual shaft stepper motor, difference none other than dual shaft has the shaft coming out the back of the motor as well as the front.
I would stay close to 20 times Volts rating of the stepper to reduce heating.