Is it possible that I can delete questions in SO which have 3 answers and some comments? Due to privacy concern, I definitely need to delete that.
Once you put something on internet, it is very hard to delete later on.
and you may think gravatar picture is not related, actually gravatar image can validate your email address like this
There are couple more things you can do, apart of deleting the question.
Disassociate the question from your account, which is explained in How do I disassociate a question with my account? and even better than just deleting, as even high rep users and moderators can't know who posted it. The end result is something like this:
Edit out the sensitive/private information then flag your own post and ask a moderator to redact the other revisions. This way you don't need to delete the question or disassociate it, whatever information you don't want to be exposed will be gone forever. (Redact means hard delete.)
You'll probably need to email
firstname.lastname@example.org and see if they're willing to do a hard delete. Mods are going to be unlikely to delete it because the presence of multiple answers means valuable content, and in any case, 10k users would still be able to see it. The dev team, however, who are the ones at the other end of that email, are able to remove the email from that post and make it look like it was never there.