The reality is that, while the third declension follows lots of different patterns, it's never that hard to identify a form when you come across it in reading if you know the regular and contracted endings. The endings are almost always the same (sing. gen. -os, dat. -i, acc. -a or sometimes -in for stems ending in a vowel; plur. nom. -es, gen. -on, dat. -si, acc. -as) except in cases of contraction, and even then it is usually pretty easy to figure out. As with so many things in ancient Greek, different authors use different forms (polis has poleo^s, poleos, pole^os, polios and poleus as genitive singulars in different authors), so it's more difficult to predict what a form will be than to recognize a form when you see it.
Smyth has the best rundown you will find (pp. 58-73), but my advice is to learn the endings well enough to recognize them, and spend your time memorizing verb forms (particularly oida and the -mi verbs).