The D3 is a USB Plug-and-Play audio device. This means that the drivers it uses are developed and provided by the developer of your computer's operating system. In the case of a Windows PC for example, these drivers are provided by Microsoft, and are a part of Windows.
D3 can easily drive headphones with impedances as low as 10 ohms.
Yes! You will need an Apple USB 3 camera adapter. All you would need to do is connect the D3 and it's USB cable to the USB A port on the adapter and then the male lightning end of the Camera Adapter to your iOS device. You will also need to have a power supply power enough to power the D3 and your device. Apple's USB C Fast charger is recommended.