Interestingly, the only mistake on this page is in your title, which should be 'Where are the mistakes?'. If we are assuming that these mistakes exist and you are asking us to locate them, then we are talking about specific mistakes and so we need the definite article. Otherwise, you'd ask a more open question such as 'Are there any mistakes?'. In this case, the answer is 'No'. All three sentences are fine.

The only mistake is in the wording of your question. You should ask, "Are there any mistakes?" "Where have I made any mistakes?"