@Warmachine: Steel is not better in every regard. For example, it takes more iron ore per construction part, and the steel plant will require more population, and cause more exhaust than a corresponding iron plant. I don't plan to make an iron-less version in the near future, but feel free to make and publish one yourself.
@DWAM: The plant will focus on what you consume. If you only consume CP3 (red), the plant will focus on making those. If you don't want to sink resources into CP1 and CP2, you can lower the yellow slider in the CP1 and CP2 output storages. This way, less CP1 and CP2 will be buffered. Note, however, that CP3 production requires CP2 and CP2 requires CP1, so the plant will need to produce CP2 as well as CP1 to make CP3.