It is unclear as to what will be the plot for season 3, although judging by the way season 2 ended and also based on the Castlevania franchise and its antagonists, it’s probably not hard to figure out who will play as the big baddie in season 3. There was no mention of when the third season will be returning, but given that there are one year gaps between the first and second season, it’s not hard to imagine that season 3 will arrive in 2019.
There is also no mention of an episode count so it is unclear if season 3 will continue with the 8-episode format or if we might be able to expect more. Castlevania’s third season was actually hinted at earlier this year where the voice actor who voices the character Trevor Belmont, Richard Armitage, said that they were about to record for the third season, so this news more or less confirms it.
So far the Castlevania series has been well-received by critics, which is surprising when you consider that most video game adaptations tend to not do too well.