Don't forget the weight. 3-days worth of stuff is differnet for many people. I would gather everything you plan to carry and weight it. Once you get to the 30lb limit, most 3-day packs don't have very good frame structure to comfortably support it. I know the old ALICE medium-sized ruck w/ frame has a 2440 cu inch capacity, and can carry up to 50 lbs...although they're not very comfortable. I can tell you that you'll have a hard time keeping any loads over 30lbs stable with the majority of 3-day packs, not too mention they lack any comfort at all for long distances. Looks for packs with at least aluminum stays. If you're looking at internal frame-type packs, make sure they have decent suspension and can compress their load for stabilty.
An excellent and long thread about assault and 3-day packs (yeah, I know it's tactically-oriented, but it has great reviews), with good reviews on several different types.
http://www.ar15.com/forums/topic.html?b=6&f=10&t=186483
My preference is Kifaru...quite expensive, but you definately get a sound system for your money; ranked with Kifaru is Arc'Teyrx (in my opinion). There are some very good (and less expensive) options from Kelty, North Face, Lowe Alpine, etc. What I like about Kifaru is the ability to go "naked" at stay within the packs ~3000 cu inch (my Zulu is 2800 and my Pointman is 3000) capacity, or you can add a few external pouches for easier organization and increased capacity. Even the simple external "pod" is great for adding that light but bulky sleeping bag or heavier clothing layers during colder seasons.
Again, decide how much gear you really need and weigh it. After that, if you have an REI or other good backpacking store nearby, you can get take a stuff bag with some heavy items (sack of beans, rice or sand) to approximate your loadout's weight and take it down to try out several designs with the weight you plan on carrying. Cinch down the pack and jump around, walk around, etc. You won't know much until you take it on a 3-5 mile hike to find hotspots, chafing straps or the true comfort of the belt, shoulder harness and suspension.
ROCK6