If you plan on doing it in the morning (because you train at night?) I would go with lower intensity fasted cardio. With some coffee, if you use it, as the only thing you consume prior to training.
If you have your heart set of HIIT, I'd do it later in day/night or even as a part of your full body workouts. If it is done in a fasted state then fat AND some muscle will be used to fuel your body. The reason is, thanks to the nature of HIIT it prefers to use carbohydrate as it's primary fuel source. One of the benefit's of this type of training is not what fuel you use during the activity but what happens after. EPOC (Excess Post-Exercise Oxygen Consumption) is through the roof and the metabolisation of fat has been shown to increase as much as 900% when compared to steady-state cardio of equivolent length.
As for diet, what does it consist of?
Is it just personal preferece to train 3 times a week?