A word accented on the third-to-last syllable is also accented on the last syllable when followed by a possessive. The rule does not apply for any word that has three syllables.
The accent is used on the syllable that is stressed. For example, in English, if the words were accented the say way, you'd say fámily, sándwich, foótball, cliché, efféct, despíte etc
So it's kinda like in Portuguese. My language used to have an accent over every stressed syllable. Nowadays, most of the words still carry it and many others have lost it. More's the pity. Whenever I see those Greek accent rules I remember those Portuguese classes about stressed syllables: Proparoxítonas (third to last), Paroxítonas (second to last) and Oxítonas (the last one) ahahah. Anyway, Ευχαριστώ πολύ.