Can't get to alternate stage 3 in Castlevania: Rondo of Blood
Solution 1:
You can only get to stage 3' from stage 2', and even then you'll have to follow a particular path. The easiest way is to jump off the bridge at a special point, where the Ferryman will take you across the lake and you'll avoid fighting the stage boss:
This video's from the PSP version of the game, but he fights the boss instead of taking the shortcut:
This page has pictures along with a text walkthrough that might help.