hi Lori. welcome to BW
I won't be of much help as DF never worked for my dd when she was your son's age, so a newborn, and we stopped it until she was 5 months or so.
anyway, you're not doing anything wrong!
At 3 weeks babies can still be behaving the same, day and night, and quite difficult to be put back to sleep once awaken. I know my dd was. we would spend from around 1 to 2 hours at night trying to put her back down once she woke up to feed. and poohing after the feed- completely normal at that age, so nothing to worry about.
BUT how do you change her? do you keep lights low and stay quiet so not to overstimulate her?
what time does she wake to feed without DF?