However, I am assuming that in the situation, there would a be a little more information for them to go on, so you could say:
Which of the three is bad? (The use of "is" would suggest that you are expecting the answer to only involve one of the three.)
Which of the three are bad? (The use of "are" suggests you expect that at least two of them would be involved in the answer.)
How many of the three are bad (people)? - The answer would simply be a number.
Are any of these people bad? If so, which (ones)? Elicits a yes/no, followed by a specific answer.
Student or Learner