First trailer for the Castlevania Netflix series!!

Cernex and others, what I was going to reply about this was just that beyond what was in the Prelude opening, in the game/manual and ending for CV3, there was no over-arching story for the series. These old titles were not made with a canon over a millennium's time in mind. Everything released in the 80s to early 90s was more or less just telling a different standalone, simple story of a different (or same: Simon) Belmont against another revived iteration of Dracula and his castle, or multiple retellings of Simon doing it.

IGA more or less gave the series a story where it had none, at least not beyond what was in those games themselves. There was no cohesive connection between them before, just a set amount of open time periods that were grabbed and had games made in, just going down the line with "son/grandson of". If you were to find and ask the creators of CV3 now, I'm fairly sure they would respond with something like they were on other projects after that and figured there would be more games to fill it out. With that going on long enough, eventually someone would have to bring them together. Similarly, I can't imagine the creators of games like Mega Man early on had any plans for how an over-arching story was going to connect them or how they would tie into future games/other series in the franchise. They were more or less just making NES/Famicom games.

Circle of the Moon's case is a little different. IGA did initially express some distaste for the game, but I've also read that the reason it wasn't included was out of respect of the creator's wishes that it be a non-canon separate entry. I don't think it necessarily had plot holes? The only significant thing like that I can think of is what happens in Legends.