I do 6-7 weekends a year, least one long trip, did a lot more in college and my light pack, a Dana Bomb, is still working fine after 8 years. Its been stitched a couple places, but due to snags and clumsiness not pack failure. Dana, Arc Teryx, Gregory, Lowe, Kelty ,amongst others, all make good stuff.
Most of the Military packs Ive seen dont seem to be real sophisticated and on the heavy side. The 3 day on Blackhawks web makes no mention of suspension, so it may have none. As long as it isnt overloaded, it will do, but wouldnt be my choice and may not be so comfortable. But I dont think you'd be anything but overloaded with that pack.
You indicated you hope not to be walking much, but given that you've got 3 gallons of water already (at 8.3 lbs plus container) thats about the total packweight I see on 3 days (20-25lbs). I dont think the pack I saw will handle 35 pounds, about the least I can figure with additions. It looks a bit small for your use as well, 2200 cu.in. doenst go far.
Maybe drop by a GOOD backpacking store. Let the folks know your parameters (weight, use, etc.) and they might be able to point out some suitable packs. I think maybe a Dana Bomb or Bridger or an Arc Teryx Bora size. They'd go about double the cost, but I'd think you'd be happier if you intended to use these for some hiking as well. Nothing worse than having a pack too small and overloaded.
As far as water, 3 gallons will go a long way for one person. Most packs have hydration pocket of some kind. Its easy enough to rig one if not. I always have Iodine tabs on me (Potable Aqua) and Vit C tabs to rid the Iodine taste. Ask about filters at the Camp store they can give you the scoop.
Luck
Alac