What you are making in essence, is a rope. A rope gets its strength from the lay of the bundles, so if you have different amounts in the bundles, the smaller bundle will have undue strain on it compared to the larger bundle. As far as the number of bundles, it doesn't matter. If you twist a good two bundle, you can make the string just as round as a three bundle. The only advantage of a three bundle is the number of colors you can use. But if you just want a hint of a color, just replace one of the strands in the bundle with the color you want to 'hint'.