The 3-year-old female dogs,not born puppy,what are the reasons?

If the puppy has some defect the mother usually eats it. Sometimes is not that what is happening. You should be at her side once the puppy is born, then you take the puppy and clean it, and clean everything around the mother. Then you try to make the puppy get some milk from the mother, try to distract the mother while doing this. After some minutes she will not have the instinct of eating the puppy. She will see it as her puppy.

If there is something wrong with the puppy she will do this, or if she feels that it is not safe or that she cannot look after it she will do this..Also she is very young and it might just be her not knowing what to do..

It happens in some kinds animal , so you'd better take the puppy away.