right now i am 13, 139lbs and 5ft2. i would like to get down to 110lbs-120lbs. i eat 3 small meals a day and i have 2 small healthy snacks e.g apricots or other fruit, plain popcorn etc and i drink 2-3L of water a day. i do a 15 minute workout by blogilates on youtube then 300 crunches and 30 minutes on tredmill. if i am busy i will just do the workout by blogilates and 100 crunches or 300 crunches and 15 minutes on the tredmill. Is this good and if i keep this up everyday will i loose 20 pounds in 3 months. i still need more tips.
I think that the diet routine/plan is a very healthy and sensible one! You don't really have to drink that much water though because, although it may be hard to believe) but too much water can be bad for you (research these reasons) so I would say just stick to 2L. I think maybe you could take out one of those exercises because if you tire yourself out too much then your body won't process nutrients from the foods that you eat.
Overall what I would say is:
Avoid drinks other than water most of the time
Make sure you eat two snacks and three decent meals a day
Don't over-do the exercise
ONCE OR TWICE A WEEK:
Have a day when you allow yourself a treat otherwise you will want to binge on these foods if you don't eat them for ages
Once you have reached your goal weight, you don't need to continue this routine as much because then you may become underweight and that can cause health issues