Yes, it is very doable.
My suggestion would be doing a little research on nutrition if you haven't already. What you eat is as important as the calories you eat. But yes, with a caloric deficit, HIIT and circuit training, 40 lbs in 3 months is attainable. I averaged 10 lbs a month when I lost 50 lbs about 8 yrs ago. I did the HIIT and Atkins. Now if I were to do it I would modify Atkins to be more Southbeach. Essentially low glycemic diet with plenty of good fats, proteins and carbs.