I don't remember there being a fight in the LN's with the 3 generals combining and Wiz+Vanir showing up. I feel like that was added to the movie if I'm not mistaken. The fight in the book ends with Komekko shooting the gun and making her introduction in spectacular fashion lol. The added movie fight was pretty fantastic too, don't get me wrong.
Sylvia going to the afterlife and seeing the other 2 generals I do remember though. And in the end, there was a scene with Kazuma asked to allot Megumin's points for her into other spells and he just buffs up her explosion like in the movie. Buuuut for whatever reason I remember that from a later novel that involves a demon king general+origin of why Megumin loves explosion magic+Chomusuke. Again, I could be remembering incorrectly as it's been some time since I read the light novels