All are correct. 1 and 3 could imply designation and/or selection, such the one he ate. The other two, I believe, are more about how than which - the cake had currants in it, that's how it was.
I am sorry. I would say both are correct (but I think your first and third sentences are more clear). I am a learner like you so please wait for a teacher.
If you are a learner, maybe it would be better to wait until a teacher replies to other people's questions. I think that way there would be no confusion.