using the A4988-drivers at 1,5A requires good cooling. According to the datasheet everything above 1A needs cooling. I would even add a fan for better cooling or stay with the current at 1A.
The power-supply has to be able to deliver the current.
So if you would like to drive all 6 steppersmotors at 1,5A per phase this means
6 steppers * 2 phases per stepper * 1,5A = 18A.
There is a voltage in the specs but this voltage is not important. For improving torque almost any stepper-motor is driven by a higher voltage in chopper-mode. The stepper-motor-driver creates a pulse-width-modulated voltage which limits the current but the higher voltage offers faster rising of the current in the coils. (inductive loads = coils) slow down current-rising higher voltage = faster current-rising = longer time with max magnetic force.
This means your power-supply has to have minimum 8V and 18A = 8 * 18 = 144W
If it is a 12V power-supply it is 12 * 18 = 216W. Your stepper-motors will consume only 65W but pull a current of 261.5 = 18A. And this is the reason why your power-supply has to be able to deliver 18A.
3.6V 1.5A means resistance 3.6V / 1.5A = 2.4 Ohm
without choppering (= PWM-voltage) each coil would pull 12V / 2.4 Ohm = 5A. Which would be too much and this is the reason why the choppering is needed to reduce the current to 1.5A. voltage is switched on for 30% of the time the rest off. 5A * 30% = 1.5A
Without choppering (done by the A4988-driver) your 62 coils would pull 62*5A = 60 Amperes
For the A4988 driver-logic you can use a DC-DC-stepdown converter like this
ground of this converter must be connected with ground of your microcontroller