Sometimes no matter how well we follow the rules and keep the chicks brooders clean and give them fresh water and food and keep the temperature just right they sometimes get sick and die. There are too many reasons to list but I DON'T think it was something you did. It was probably something the little guy/gal was born with.
I'm really sorry for your loss. I know it doesn't make the hurt go away but almost all of us have lost a chicken and we know how much it hurts.
Take real good care of the chicks you still have and watch them VERY carefully. Let us know if there are any changes in the way they act or eat or drink or anything that seems unusual. Sometimes chicks can get sick but there are medicines that they can take that will help them get better.
Sorry about your loss. 2 years ago when we first got chick's, I had to put two down that appeared very healthy for the first 3 and 5 day's. I took it upon myself after asking for advice here to put the first one down. It was very hard and not easy at all, the second time was still very hard, Hate losing the babies. Was glad that the rest (26) did well and grew up to be healthy hens. I lost one of our chicks this year that appeared healthy when we got it at the feed store. Little thing passed in my hands, It just happens, do the best that you can and the strong ones will survive. Good Luck!
Meat birds are pretty frail creatures. They are so specially bred to gain meat as quickly as possible that they are not hardy in the least. They have health problems throughout their lives that frequently lead to their early demise. It could have died for any of a hundred reasons. At three days old I would say that the bird was suffering from "failure to thrive" which is a catch-all term meaning the bird couldn't live due to a congenital, genetic or hatch defect. It happens sometimes.