My company designs/builds medical devices so we validate our process for constructing the devices. I'm wondering about our sample size, (3), and if it makes sense.

The process consists of several fixtures to help with assembly, to program firmware, etc. The validation for each sub-process consists of doing an IQ or IQ/OQ or IQ/OQ/PQ for each fixture. Our sample size for the PQ is typically 3, because we only build 3 devices (its a large, expensive device). The PQ is successful if all three devices meet performance requirements.

I''m not sure about the statistical rationale behind the sample size of 3, but also I don't think we can justify building more than 3 systems to validate the processes. Does anyone have guidance for this situation?