"Some people feel the 3-ply is rounder. But if you make the 2-ply properly I don't see any advantage."
Yep--at least that's the only "advantage" I ever hear about...well, that and being able to use 3 separate colors.
Making the 3-bundle is a little more complicated--you have 3 bundles to keep even tension on vs. 2.
Suppossedly the rounder loop in the 3-bundle is easier on your bow. I've seen good arguments "on paper", but not in real life. Like Guy said, if you know how to make a string properly, there aren't any real advantages to a three bundle over a two bundle--at least none I know of.
I use my old longbow as an example. It's been my favorite for 15 years. It's NEVER had a dacron string on it, and rarely has it had a 3-bundle string on it. It's never had an endless string on it.
If a 2-bundle were going to damage it, or if "FF" were going to shorten it's life, or if a flemish string were detrimental (I've seen all this and more purported); I'm positive that if any of that were so, I'd have seen some evidence of it by now. Other than some scratches and dings (been refinished twice, will need it again sooner or later--it gets used) she's as good as the day I got her.
Anyhow.....
It's not a bad thing to know how to make a 3-bundle, but you won't be missing out on anything important if you don't.
Also like Guy said, it's easier when you use 3 colors. Make a sequence, and keep it for both loops. For instance, if you start out your twist with red/white/blue, maintain that sequence throughout the string. Make the first loop, keep the bundles in line and in sequence, and make the second loop the same way. You can do it with one color, but two or three is much easier.
Chad