triple water bottle lumbar pack?

No, but two options that come to mind:

1. Use a bladder in place of one of the bottles and stick it in the main compartment of your lumbar pack. If you connect a hydration tube, you even get "hands free" hydration (if that's what you want). It's what I do when I need to haul 3-5 quarts of water.

2. Lash the third bottle horizontally onto the outside of your main compartment — with cord or velcro.

I've found that I never need to have more than two water bottles accessible at any moment. I normally keep one bottle of water in one side, and one bottle of Gatorade on the other side. Then inside the lumbar pack, I will have a third bottle. It can fit one or two more if necessary. However, that is going to be a lot of weight bouncing around on your hips.

–B.G.–

"However, that is going to be a lot of weight bouncing around on your hips."

Not necessarily. I use Platypus bottles/bladders. These being soft sided containers, I squeeze out the excess air after filling. Carrying 5 liters (as posted above) and connecting one of them to my hydration tube — I can feel the weight (although still quite manageable) but not any bouncing around at all.

Benjamin, I did not say "splashing around."

You will have the same weight of water in your pack regardless of hard bottles or Platypus or anything else. Whether that weight is bouncing or not is purely a function of the stiffness of the lumbar pack. On many, the connection between the actual pack compartment and the waist straps is not stiff at all.

–B.G.–