The first one seems wrong not just grammatically but also from the implied meaning. Even if you were to use a sentence beginning with 'Because', using it right after a question indirectly implies that it is related to the answer. But here, you intend to convey part of your question through that clause.
The third one is wrong since the question mark does not end the sentence as @Peter_Shor has commented. In another sense, it is a less punctuated version of the first one as well.
This leaves you with the second, which seems about right.
Tip: The only times you'd see a question mark before the end of a sentence is when there is a quotation in the sentence or when the sentence is a question-exclamation; eg:
"Why did he do it?" asked the teacher.
Isn't it shocking to learn about the incident?!