I assume the question is about MWMT. Unless your group is VERY slow, you should't be carrying more than a liter of water per person until the last source (switchbacks 23-25). There is no point of carrying extra weight when you can always stop, hydrate and refill - there are plenty of streams along the way. At the last source, hydrate well and fill up as much as you need, depending on the rate of consumption and pace. That being said, even for a slow party 3 liters per person is a bit too much. As a frame of reference, I am 190 lbs and I usually bring 2 liters for the whole night when I sleep on the summit.
Edit to add: The best hydration system that virtually everyone uses on long distance hikes is comprised of a SMART water bottle and Sawyer filter (I personally like the regular version but mini also works). Get two bottles and alternate the filter - it fits perfectly with enclosed ring. You don't need a squeeze bag. The system is simple, durable and easy to use.
