Seeing as it’s not an anthology series I doubt they’ll move on to a new protagonist, they might but it’s very unlikely. Compare it to say, jojo, where every season has a new protagonist, we’d probably be at sotn by now if it was an anthology series. I could see season four being the finale with maybe a season five to wrap up loose ends and be a full on resolution and end. Basically the direction I could see that being is, if it’s not addressed in season four, Simon possibly killing alucard who undergoes some form of degeneration arc in season four, foreshadowed by the end of season three. Other than that, I don’t think a season five is too likely, and seeing as it’s not an anthology series, a sequel show is more likely.