Since you really don't know if it will be raining, sleeting or snowing or just very cold when you need your 3 day bag, I am going to suggest something a little different that I have kept in my vehicle for a while and pulled out on occation.
I don't see exactly what I have at Cabelas, but they do have a Cabelas Gore-tex Exv Parka and they you buy the liner seperatly.
It doesn't pack small, but it really does the trick.
If you are just looking for something that packs small and keeps you mostly dry, I would suggest a garbage bag and a boonie hat. The arms will get wet, but your core will be dry.
One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine.