My daughter is 19years old and eats three square meals a day but doesn't seem to put weight on . what can please be the cause.?

Depends on activity: At 19, i would expect her to not be growing in height any more. If she is a healthy weight, and has an average diet, there is no reason that she should put weight on. At her age, her height and weight, if in a healthy range, should remain constant. Are you concerned that she is underweight? If that is the case, then she would need to consume extra calories to gain a little.