CV3's difficulty could be in the branching paths and length of the game. You don't really play the same stages over and over again unless you do the same route each time. I've never beaten the game, though someday soon I'd like to hunker down and just go at it.
The first Castlevania I find to be a lot easier and can beat that birch quickly. Of course, to do it quickly requires me to hang on to the holy water for the last couple levels, lol, since that's the cheapest of the cheap subweapon in that game. But even without, it doesn't take a terribly long time to do the last couple stages.
It's a case where familiarity has caused the game to become easy; memorize the enemy patterns and it's a cinch. I've yet to play CV3 enough times to become that familiar with it.