your daughter is healthy. don't worry. my son came out 2.5kg and by the time he was 3 months, he was hardly 5kg. as long as her diapers are full which means she is frequently urinating and passing motion then it's ok.
I came across this post. My baby is 3 months and 17 days today. She is about 5.2kg. Her formula milk consumption is about 450ml - 500ml a day. I tried to feed her more but she refused.