My baby is 3 weeks old, I feel my milk is not enough, is it fine to give her bottle milk as she feels hungry, also she is very thiny, is she ok.?

Formula may help. At 3 weeks of age a full term baby should have gained about 12 oz over birth weight. Some mothers don't produce enough milk for a baby to grow. A lactation nurse may be able to help. Sometimes breast feeding, then giving formula will fill up the baby and give enough nutrition to gain weight. You should call your baby's doctor for an appointment.