Forget about your metabolism dropping and going into starvation mode - that's bioscience.
However, please note that you can do those diets for very short terms only. Trust me. Also, some of that weight loss will be from your glycogen stores. Probably half of it.
if you diet like that for longer than 2-4 weeks, your hormones will get screwed up and you will more than likely start binging. I tried to keep a 1000 calorie deficit for more than 4 weeks and kept trying until I always wanted to eat everything in sight.
If you want to see continued progress in your physique, I would start strength training 3x a week, doing some cardio and up your calories a bit otherwise you'll just get 'skinny-fat'. You can't expect to lose 3lbs a week - it is unsustainable and you'll be losing muscle quickly!
Hope that helps! Keep it up.