I'm presuming that there was a schema version 1 based on the naming of the version 2 tables (they all end in '2'), but this may be incorrect. In any case, schema version 2 corresponds approximately to SMW v1.x, and schema version 3 to SMW 2.x (and some high-numbered 1.x beta releases).
schema version 3
These are present in SMW version 1.9 alpha:
schema version 2
These were present in SMW version 1.7, and may also be valid for 1.8 (but not 1.9):
Core tables are boldfaced; the others seem to be primarily for optimization or auxiliary purposes.