I finished watching season 3 of Highschool DxD and I want to read the LN. I don't know where to start reading. How much did the Highschool DxD anime adapt the light novel, and did the anime adapt the LN well?
Season 1: Volumes 1 and 2
Season 2: Volumes 3 and 4
Season 3 episodes 1-9: Volumes 5,6 and 7
Season 3 episodes 10-12: Original
Ovas and Specials: Half of volume 8 and original
The most likely future:
Season 4: Volumes 9 and 10. The way these 2 volumes work it should be this way
I watched the anime and read the light novels.
About your second question yes, it was very well adapted. Season 3 was a mess though so I'd advise you to read it. Just for Koneko it's worth it. It's nice having her fall for Issei and enter mating season. Koneko in the heat is the best Koneko :)
Anime covered about 7 first volumes.
Based on Manga-Updates