I'm building a single-tier with high pressure, too. Because most of the flexible hoses at the local box stores are low pressure, I'm also going to be using flexible copper from the valve (high pressure) to the burners, too. The valves only have 1/4" out, so I'm inclined to think 3/8" copper tubing should be fine. My initial tests indicate that there's plenty of output through that tubing. As far as firing all three goes, I wouldn't be too concerned. I doubt you'd be firing all three at wide open concurrently. In fact, I can't imagine having two on simultaneously that much in a normal brew day.