This SHOULD be sufficient to run the HDD, provided it is designed to run from USB with the 500mA limit this imposes.
Unfortunately it may not be reliable, as it depends on the voltage. My Pi2 power supply, rated at 2A, drops below 4.8V when I tried running a HDD, and while this is in spec proved unreliable, so I use a powered hub. There are a number of items in the path, the polyfuse and the USB supply regulator which contribute to the drop.
You need to give it a try, and measure the actual voltages.