You are correct. ScintillaNET v3.x is vastly different from v2.x. It is a complete rewrite. There has been no attempt to keep the API compatible and no plans to create any kind of "compatibility layer ...

ScintillaNET is a Windows Forms control, wrapper, and bindings for the versatile Scintilla source code editing component. "As well as features found in standard text editing components, Scintilla ...