How much formula for 3 week old 7lb 8 oz baby?

My three week old seems to want more than 3 ounces of formula during her feedings. She weighs 7 lbs 8 ounces now. I have read that she should be taking 2-3 ounces every 3-4 hours, but she is consistently acting like she wants more. I don't want to make her sick by giving her 4 ounces. How much do you feed your 3 week old?? Thanks. PS feeding did not work for us :(