The Culprit could be:

C

logic:

A says: I did not do it [TRUE]. B did it [FALSE].
B says: I did not do it [TRUE]. I know that C did it [FALSE](just a guessed).
C says: I did not do it [FALSE](which means C's the culprit). B does not know who it was [TRUE](which means that B was just guessing).

The Culprit could be:

C

logic:

A says: I did not do it [TRUE]. B did it [FALSE].
B says: I did not do it [TRUE]. I know that C did it [FALSE](just a guess).
C says: I did not do it [FALSE](which means C's the culprit). B does not know who it was [TRUE](which means that B was just guessing).

The Culprit could be:

C

logic:

A says: I did not do it [TRUE]. B did it [FALSE].

B says: I did not do it [TRUE]. I know that C did it [FALSE](just a guessed).

C says: I did not do it [FALSE](which means C's the culprit). B does not know who it was [TRUE](which means that B was just guessing).

The Culprit could be:

C

logic:

A says: I did not do it [TRUE]. B did it [FALSE].
B says: I did not do it [TRUE]. I know that C did it [FALSE](just a guessed).
C says: I did not do it [FALSE](which means C's the culprit). B does not know who it was [TRUE](which means that B was just guessing).