3lbs is pretty small still, so I agree with Cynthia that it might be Poppy's natural limit for now. She'll definitely eat more as she grows and if you still have doubts though, you can try her on a different brand to see if there's any difference in her appetite. Sometimes puppies need time to transition between brands. However if you notice lethargic behaviour / low energy it's definitely a good idea to consult with a vet.