Hi amayilsamy, using darlington pairs for this ammount of current is almost prohibitive, unless you use a chopping technique, for example, suppose you have a darlington with Vcesat=0.9V (I haven't see none with this specs), when any coil is energized and consuming 3.0A you will have a power loss of 0.9V*3A=2.7W per transistor. If you use a full step wave drive you will waste 5.4W as heat. Also, using this configuration when the stepper stays at detent position the current will not be controlled, unless you use a 3.0A capable power supply. It's true that this method is expensive but if you have the drivers then perfect. To end this post this driver also includes a translator so for stepping you only need to send pulses. I agree with you about the chip, should be used a new one.
Best regards, Yosmany325.