While I would say the third of your options, "non-defect-source-assesment processes", is most correct, I would strongly suggest trying to rephrase the subject for clarity. The hyphens can be used to indicate at what level the negation applies, so in this case "defect source assessment" is being negated, but "processes" is not. This is appropriate because you are talking about processes (not non-processes). In the second example, "non-defect source assessment processes," the implication is that "defect" is being negated and "source assessment processes" is not.
The problem is that punctuation is sometimes a bit more subtle than you can expect your readers to understand. If you don't want them to get confused, "processes not related to defect source assessment" might be a clearer noun phrase.