I hv 3 meals a day ..does any one have any suggestion
6 meals a day and get checked for diabetes. Or if already dx as diabetic, work with your clinics' dietitian for several sessions until you can figure out how to down the requisite calories while controlling your blood sugar.
How many calories do you eat in an average day? If you don't know, track your consumption for a week. Then try to match a calorie goal you developed with your dietitian (in the mean time you could just increase intake by 500-1000 calories). My dietitian recommended 3500-3800 calories a day for me, if you want a very rough ballpark comparison (but I'm not trying to put on weight, and I'm a bit older than you, so I'd guess your needs would be higher).
Really finding 1 or 2 good staple foods that you can consume in large volumes, digest well, and are very easy to prepare/access seems to work for me to hit my calories needs. Cereal, Sandwiches (grilled cheese or PB&J), trail mix (nuts, dried fruit, granola), or something else that works for you. I tried shakes for a while, but I just couldn't drink enough of them without getting appetite fatigue. It also helps that my wife will prepare foods in large enough volumes that I can eat big meals for dinner and then take big left-over portions for lunch the next day. With CF always cook large portions, save the extras (if any) and reheat them whenever you have any sort of appetite.