For a three day backpacking trip in the summer then a couple of pairs of thin poly-pro sock liners and trail runners will suffice for most trips.

For a 3-day backpacking trip in the winter, then that is a completely different beast. I tend to get really cold feet and hands and thus it is not uncommon for me to wear two pair of liners plus a wool sock to keep my feet warm even when moving. I also wear a pair of Keen's Summit County Winter boots that are rated to -35. When I am in camp I also use down-boots to keep my feet warm when I am inactive.

In each case I will always bring one pair of "sacred" socks that I only use to sleep in and on wash days. That way I know I will always have one pair of clean, dry socks to wear during the night in my bag.