How do I know?

My baby is 3weeks old , and I started giving her 3/12 oz I started noticing when she's done shell stay wide awake and I'm not sure to feed her again or if it'll be over feeding? When do I start giving my baby more milk , I feed her every 3 hrs sometimes I'll go the 2hrs if she's awake still , ftm and I don't want feel like I'll over feed her bc in her 2week check up she did gain weight and doc said only 3 oz , but she seems to be hungry after a feeding still and she's only on my milk