The simply answer probably lies in the fact that you have the Priority field on the PartSources all set to the same thing. While RapidResponse will always designate one as a "Primary Source" it will still plan evenly amongst all PartSources with the same Priority given the effectivity of all are equal. I would suggest trying that first. By setting one of the three PartSources to a Priority of 1 and the other to 2 and 3, the planning should use only the Priority 1 PartSource. If that has the desired effect then problem solved and you don't have to omit importing the other sources.
If your intention is to use the other two PartSources in certain cases then it probably involves a more detailed discussion of the requirements.
Let me know if this solves the problem.