My son has a pit bull , that gave birth about 3weeks ago, she seems like she's
Losing too much weight, what can I do to have her gain weight?

Nursing dogs need more food to nourish themselves and the puppies, so give her more food than you would normally. Also, you may want to start her on a vitamin for nursing dogs. If she has trouble because she may have too many pups for her body to take care of so you may also want to consider getting a milk replacer and bottle feed some. Of course alternate pups so that everyone gets some of mothers milk.

It is normal for the mom to lose some weight and need to eat more than usual. Puppy food is a good way to increase her calorie intake and meet her nutritional needs during this exhausting time for mom. I suggest sticking to her normal brand when going to puppy food so that it doesn't through her system off.