As a quick response, I would refer you to the paper that I presented at the 2013 Annual Reliability and Maintainability Symposium (RAMS) entitled “When Good Requirements Turn Bad”. It uses a study performed on military electronics equipment to identify the distribution of 8 separate causes of failure (including the 3 that you mentioned) as a starting point for determining the “correct” inherent reliability requirement to specify on contract in order to achieve the desired operational reliability in the field.

Note that the percentage for each failure cause type in the overall distribution can (and likely will) vary based on a number of factors, including type/complexity of equipment, the environment it’s used in, the technologies involved, the skills of the operator/maintenance personnel, etc. Therefore, it is preferable for each organization or program to determine its own “unique” distributions, if possible. Distributions such as the one in the referenced paper should only be used as a guideline in the absence of other data, and then modified based on “your” experience.