So problem i guess quite common. I watched Overlord till 3rd season some time ago and decided to read it. Where should i start? Also i think there is LN and WN. Mostly prefer not to reread story but I can back up a bit.
bro i tried asking once, they all said reread it from the start, RIP You, I wish you the best of luck finding it,
When they adapt a LN into an anime it's generaly 3 volumes each season. So season 3 should end with volume 9. Tho I do recomend quicly reading over the last chapter to be sure of it. If I remember right the epilouge of volume 9 is also not in the anime. I do recomend reading from the start, but I can't realy do that when I skiped volume 2-6 cause I had just seen season 1 and 2...
You should read the LN, as the WN has a lot of differences that include new characters (basically almost all of the Guardians, just to give an example, since the WN has only Shalltear and Cocytus) as well as some characters that, due to their role being taken, were killed in the LN/manga/anime version. If you don't want to reread, since I believe the anime covered everything that skythewood translated, then you should start at volume 10.