Have you ever heard of frugal squirrles? Go there.
http://www.frugalsquirrels.com I am in the forums, but mainly I lurk there. VERY good information. What you are wanting is called a BOB (bug out bag) or 72 hour kit. The reasoning is so you can hike to your bug-out place or get home to your bug-in place. Or, as I am wanting to use it for, to randomly decide to camp somewhere for the weekend and be able to do that.
Search the frugalsquirrel forum for BOB or other stuff. Better than that, just read the different forums! There is a LOT of information out there. You are going to want to keep it fairly light (most people try to keep under 60 pounds, I am a 135 pound guy, that would weigh me down a LOT, I would aim for 20 pounds or so).
A list of stuff I would include:
6 MRE's, 6 packs of Ramen (taste isn't a real big deal for short term stuff), some fruits and sugars, etc.
WATER (or a means of obtaining this)! MRE's need a lot of water so they don't dehydrate you!!! Being able to carry a gallon would be great, being able to purify some would be awesome. You'll probably want a purifyer (if you want to go camping, you can use it for that), or if you don't want to spend that much, purifying tabs and a can to boil the water in. Bleach also works.
A tarp (6X8 or larger), orange, or white, or brown, depending on what you want. Orange if you want to be found, white to blend in with snow, brown to blend in better with ground. The tarp serves many purposes. I am taking a tarp this weekend to use as a tent (I'll let you know how it goes), you can use it over the ground so you don't get damp if the ground is damp, you can fill it full of leaves and pine needles for insulation, you can make a roof for a snow shelter, etc. If it doesn't have grommets, no big deal. What you do for attatchment points without grommets is take a small rock (inch around or so) and put it on the inside. Then tie a rope around the rock on the outside, viola, instant tie point!
Rope! A very versatile item. I'd say like 20 feet MINIMUM of some 1/4 inch cord or so. MANY people like to have 20-50 feet of parachute cord because you can take out the 7 inner strands and use them for snares or other things.
Knife. A decent one, doesn't really matter what brand. Serrations would be nice for cutting rope, make sure you have at least an inch and a half or so of blade. Keep a small sharpener.
Fire! MULTIPLE ways to make it! A couple lighers, some matches (dipped in parafin for water-proofing), fresnel lense (sunny days only), flint and steel, etc. Also have candles! B-Day candles are great for getting a fire going. Larger, longer burning candles can keeep a snow shelter at around 32 or so even if it is REAL cold outside. Also, you can drape the tarp (or a rain jacket) over you and your knees. Then, dig a small pit (9 inches across, 4 inches or so deep) and build a small fire in it, or a larger fire and scrape the coals in that spot, or put the candle in it, put your tarp or mini-tent over it, and you will stay pretty warm. You do NOT want large flames! They will burn you as you are sitting over the hole. If the coals are too hot, you can put a little dirt or something over part of it so less can combust at a time.
Clothes Some to keep warm, extra socks and underwear in case the ones you (I hope) have on get wet. Wet underwear and socks=bad. Thin and lighter is better than bulky. Layering will keep you warmer for the weight than a large coat. Longjohns would be nice.
Can opener if you have canned food.
Some other stuff that I might add if I can think of it.
So yeah, here is a SLIM basis, I'd suggest checking out that site I showed you. They are a lot more helpful than me (as they actually can DO all this stuff, being a poor college student keeps me from buying much at all).
Also, ROTATE your food! Don't want it to go bad.
If you are planning on possibly staying longer, include snare wire for small animals (learn how to use it and practice). With the shorter time you can pack canned goods, but they weigh a lot. It isn't a big deal for 3 days worth of food.
Make sure whatever you put together gets tried out. A lot of people will put a small journal and pen in their pack so they can go camping with it and write down what worked and what didn't. They then augment the pack from that.
Oh yeah, forgot lights! You don't want a lot of weight, so, buy a 4 pack of lithium AA's (for longevity in storage and cold weather performance, a very LARGE factor). I'd use the opalec/mag and have 4 AA lithiums, gives you 16 hours of light, should be MORE than enough for 3 days. For longer distances, maybe a Tec40 with lithiums and standard bulb, maybe a .75 amp one the Mag 4D cell uses for brighter, but less runtime.
Spud