Thanks for reaching out!
It sounds like you're on the right track with the Mystery Ranch Scree 32 Pack. While a pack that size would normally be fairly small for a three day trip, if you're not carrying food, water, tent, stove, and fuel in the pack then that size should work just fine. We would advise you to be sure that your sleeping bag will fit, that will likely take up the most room in the pack. One great feature of the Mystery Ranch Scree 32 pack is that is has the ability to hook and strap on other items (like an axe) if needed. While you don't want to overload the pack with too much weight (the frame is a sheet of HDPE), it is nice to have the option to add gear if necessary.
If you're concerned about space, we encourage you to check out a couple of other packs that will give you a bit more room:
When you get up above the 40 liter size in a backpack, however, it becomes pretty large for day trips or travel, if that is something you are considering. We did notice that one of the reviewers of the Mystery Ranch pack said they appreciated how it fit easily in the overhead compartment of an airplane.
Hopefully this helps, thanks!