Hi,

voltage primarily depends on the stepper drivers you use. Voltage should be as high as possible (within the drivers specification) because voltage is responsible for acceleration and maximum feed.
Current depends on the motor - 5A is a little scarce for three 4A motors - but it may work.

So: if your stepper drivers accept 48V get a 48V supply - if they accept 80V get an 80V supply.
You can try 5A if it has short circuit and thermal protection - if it shuts down just get a second one and supply 2 motors with the first and the 3rd with the second.

Christian