If you are doing sections arches, just don't decide on the sections upfront. I believe 3 colors (Ican at least) will require at least 1" PVC to get close to the semi-standard 18" per channel. I would make a pre-wrapped string (I still call them Megastrands) and wrap a piece of 1" with it. This will help you determine the size of each section. I know this because I'm working on Fire Sticks/Light Poles in the same way. I actually want the fire poles a bit shorter because I want them to be around 10 ft. for 8 channels per color. I'm doing two colors.
Of course, its very easiy to spread them out a bit to make fill the space. It's very hard to compress them to make the length shorter. PVC is cheap, by a few different sizes.
Also, I wouldn't worry about the thing about the bulbs lining up or not. I haven never seen a string of lights, either Incan or LED, cheap big box or high-end commercial, where the space between bulbs was that consistent. Mine are Random city and they look great. With a 3-color string you really aren't going to have a choice. Our first year, my son started making the Megastrings. He made 12 of them while I was traveling. I came home and bitched him because the bulbs did not line up at the end of the string. In fact they stopped lining up about two feet into it. All the same brand bought the same year at the same time. I quickly discovered it was nothing he did. Now when I make megastrings. I stagger them from the beginning.