If your Fedex Tracking information isn't updating for 3 days it likely could just be that your package was not scanned in those 3 days because it could just be traveling on a semi truck through some States to get to your location.
I find that sometimes the tracking information on some packages doesn't update on Fedex for me either for around 3 days and then during the 4th day it usually updates.
Sometimes a package doesn't get scanned again until it reaches it's destination either so I wouldn't worry about it.
But if you're worried you could contact Fedex customer service to see what's up with your package.