As you grapple with the inherent “irregularities” of Greek 3rd Declension nouns, I would strongly encourage you to be diligent to memorize the 3rd Declension Greek noun endings from this comprehensive nouns chart, just as you would the other two (easier?) declensions with their various genders and/or variations (i.e., 1st Declension). If you will do that, you will find that they are easier to identify in context, and the reasons for their various “irregularities” will be much easier to comprehend. The following discussion and related charts (click for larger, clearer view) should also provide some study help visuals.
Note: The two pages imaged above discussing and charting the declension of various types of 3rd Declension nouns are also available anytime from the sidebar as a downloadable PDF.
Go to: Wermuth’s GREEKBOOK.com