Daily milk intake

My bub is currently 3months and 24days. On average, she only take 500ml to 600ml of bm per day. She usually takes ard 25mins to finish 100ml at 3hourly. I'll need to dream feed her twice to make up the daily intake. Is she underfed? Any ways to make her drink more milk? P/s: her birth weight is 3.3kg and she's now 6kg. Weight been stagnant for 2weeks.