Three liters of water on your back is going to be really heavy, but I could tell you firsthand about running out of water on a trail.

I have a Dakine bag and I think it holds something like a liter, maybe a little bit more. I've used it for about two years and the water bladder sprung a leak so it's been chucked.

We were on the trail in about 98 degree heat a few years back and both of us ran out of water. That wasn't good, considering we were quite far from the car and probably halfway through the ride. We found a "short cut" back to the car and this "short cut" wound up being through a field which was over two miles long and was quite the climb. Normally, this wouldn't have been much of an issue but it was out in the open with no protection from the sun. I nearly had a heatstroke out there. I started getting dizzy, feeling nauseous, etc. It wasn't good. Since that experience, I don't go out in extreme temperatures and if I do, I make sure the area I'm going to be riding in will be shaded. Hell, I never felt overly great while riding in hot weather anyway, so I don't feel like I'm missing out.

Before I get off on a babbling tangent, I'll just say the experience scared the hell outta me. I seriously thought I was going to die out there. Just whatever you decide on, make sure to have plenty of water. Three liters may be a lot but the additional weight of a three liter offsets possibly dying on a trail. Maybe get a hydrapak that carries two liters and then carry two bottles as well...