I have got a same answer to three similar questions on ask ubuntu. Should I answer all three of them (answer is the same) or should I answer one and give a link of the first answer to the other two questions?
If your answer applies to multiple questions because the questions are really asking the same thing, then you only need to post one answer.
Post it on whichever question you think is the most well-established or useful (usually the older, or the one with the most views, or the most answers, or the best-written, or the least specific--this is not an exact science).
Flag the other questions as duplicates:
- flag > It doesn't belong here, or is a duplicate...
If you want, you can also post a comment on a question you flag. Usually this comment should just refer people to the other question. If you don't think people would see your answer or understand its significance to their problem, then you can include that (or whatever other information you want) in your comment.
But your comment should not answer the question (except in the sense of explaining how the other answer applies, if necessary)--it should refer them to the other question where they can get an answer.
If a large amount of explanation is necessary to make it clear to someone how their problem is really the same as someone else's problem, then it is probably really not the same, except at a deep, technical level. In these situations, it's often best not to close as a duplicate. You can post a very different answer (since answers should explain themselves when possible).
By the way, you should flag duplicates even when you don't have an answer.