Why does the Pasuk not write "u'vi'Shelosh-Esrei Shanah", with a 'Veis (like it does by "u'be'Arba-Esrei Shanah"?
Rashi: Because it refers (not to the thirteenth year of their subservience, but) to a new period of thirteen years that the rebellion lasted; 1 whereas the fourteenth year refers to the fourteenth year of the rebellion.
Why does the Torah mention the fact that the five kings served Kedorla'omer for twelve years?
Seforno: This is in fact, a continuation of the previous Pasuk, which informs us that Amrafel and the two warring factions met in the valley of Sidim and arrived at a compromise - namely, that the five kings should serve the three. 1 And it was in the thirteenth year that they rebelled. 2
רש"י: עבדו- חמשה מלכים הללו את כדרלעומר: ממה בא רש"י להוציא?
גור אריה: אל תטעה שהכוונה לכל המלכים שהוזכרו בפסוק ג [וגם ארבעת המלכים בכלל], אלא רק חמשת המלכים מרדו.