for cnc use , i prefer old fashioned transformers , unless they are small steppers , for say a foam cutter , don't use switch mode power supplys , and be careful of pc power supplys , some of them need an additional mods to work satisfactory , just a standard transformer , diode bridge and capacitors as a guide , you need to be able to produce between 24 - 40v @ 10 - 20A

do not take the stepper voltage as a guide to power supply voltage ,
add the current rating of all the steppers and you need a supply that can give as a minium 2/3's of this
3 steppers @ 4A Each = 12A , so as a absolute minium you need an 8A supply