Watch your diet/do cardio
You need to watch your diet and burn fat calories. You stomach will get bigger with ab exercises if you aren't reducing calories at the same time because your muscles will get bigger underneath the fat.
You need to do both to get fast results. But you will lose body fat everywhere, not just your stomach.
You shouldn't do HIIT three times a week. You should do longer, steady-state cardio and low-intensity cardio too. Don't do cardio more than three times a week, unless you're adding in a little after your weight workouts. You need to lift about three times a week.
Definitely watch your calories! You will lose twice as fast by eating right.
Remember, this is a lifestyle change, and you will most likely gain everything back plus more if this is just a fast loss with no lifestyle change.
The slow and steady always wins out over the fast and sporadic. You shouldn't be losing more than 1-2 pounds a week. Chances are that you will lose less than that--.5 lbs a week. This is GOOD progress. It shouldn't happen faster than that.
An hour a day, 6 days a week is fine.
And not knowing your situation, cut out all sugary/calorie-laden beverages including soda, alcohol (only in moderation), and juices. Eat fruits instead of juice, sugar-free or water instead of soda. Beverages are THE WORST for putting on pounds, along with fast-food.
You need to let your body acclimate--make each session a little longer or harder each week, if even by a few minutes, no longer than 2 hours.