If one of them had cheated

1) We did not know if someone had cheated in the exam.

2) We did not know if a student had cheated in the exam.

3) We did not know if one of the students had cheated in the exam.

Which could be used if we suspected one specific student of cheating?

They could all be used, I suppose, if we didn't have a specific suspect, but thought that someone might have cheated.

Gratefully,

Navi