Can I use the HD3 with my iPad or iPhone via USB to bypass the internal DAC and get better sound?

Absolutely! This is especially useful for connecting wired with newer iPhones that do not have a headphone jack output. You will need to pick up the USB Camera Adapter from Apple, which allows a USB connection to the iPad (and, as of iOS7, the iPhone). Then just hook up as you would from a computer. If you'd like to be able to charge your iPhone or iPad while you use them like this, you can use the USB 3 version of Apple's USB camera adapter, which also allows passthrough charging at the same time.