Hello - does you girl eat well? What do you feed her on and how many times a day. Does she look good? What does your vet say. And is she a pure bred Minpin and were her dam and sire small? And are you worried that maybe she is smaller than you thought she would be? I have a half Dachshund/Minpin and she weighed a bit more than that at that age but her dam was a very large standard Doxi. And my girl was raw fed once I had finished bottle feeding and weaning her. She is now just over 4 years old and her weight has not really changed since she was 6 months old - somewhere plus or minus 6 kilos, the maximum being maybe 6.7kg - probably less now as her weight fluctuates. Check with your vet if you are worried, and maybe her food might need to be changed?
I would always go by look and feel, to judge whether the weight of my puppy was right for her. She needs to be well covered, but elegant and with a discernable waist when viewed from above.. I don't know where you are located, but the KC breed standard does not provide a guide weight for Min Pins, only a height of 10 - 12 inches. The AKC standard says 10 - 12.5 inches, but also provides a weight of 8 - 10 pounds.
3.3kg is about 7.275 pounds [I had to google it as I only go by kilograms]. So I do not think that your girl is underweight for her age. If she looks good, if she eats well, I would not worry. Just do not let her gain too much. Do you please have any photographs of your girl for us?