If I understand correctly you are asking about posting the same explanatory answer to two different questions.
I do not see anything wrong with this, even if the two answers are identical. It is possible for two different phenomena to have the same explanation, although you should try to tailor your answer to address the specific issues raised in each question.
As I think Rob is suggesting, you could post a brief answer on one question which refers readers to your detailed answer on another question. However the brief answer would need to be sufficiently self-contained to avoid being deleted as a link-only answer. If the link were to be broken then the link-only answer would become meaningless.
Possibly the two phenomena have different explanations, in which case at least one of your duplicate answers will be wrong and is likely to be down-voted. But a wrong answer would not be deleted by the community, only by the person posting it.
If the two phenomena are the same then one question is a duplicate of the other and would usually be closed. Site policy here seems to be that you should not post an answer if you are aware that the question is likely to be closed. For example if there are already 3 or 4 closed votes, and especially if you have yourself voted to close the question, for whatever reason.
It is acceptable for the same user to post two different answers to the same question, for example proposing two different explanations. But if these two answers gave the same explanation, even if they are not identical, then one of them might be flagged for deletion by a moderator. It seems to be acceptable also for the same user to post two or more different answers to the same question which address different aspects of the question.
Quite often different users post essentially the same answer to the same question. This is often unavoidable and neither answer would be deleted for this reason alone. Such a duplicate answer which was posted hours later might attract down-votes for failing to take note of the earlier answer or to acknowledge ideas borrowed from the earlier answer, but it would not be deleted. Conversely a better answer posted later would not trigger the deletion of an inferior earlier answer.
As to the issue of whether the 3rd question is a duplicate of the 1st, I think it is. I agree with voters that all 3 questions are duplicates of each other.
My reason is that all 3 questions are asking about an everyday-life phenomenon which could quite possibly contain several features which have slightly different - or possibly completely different - explanations.
The users who posted these questions have used different words to describe what puzzles them (chain shape, wavy stream, sort of helical) but they failed to make any distinction between subtly different features of the same phenomenon. I do not think they were intending to make any such distinction.
If the question were a purely abstract one then a particular feature could be singled out in the conditions imposed by the question-setter. Those different features with different explanations would distinguish them as separate questions asking about separate issues. However in this case the questioners do not seem to be interested in any particular feature other than the general waviness of the flow while liquid is poured from a container of some sort.
Consequently I think it is up to those who answer these questions to distinguish between the different features of the wavy flow and propose different explanations for them. As I have suggested above, different answers to the same question could legitimately address different features of the everyday-life phenomenon.
Therefore because I think these 3 questions are essentially the same I do not think your answer to helical flow from cup has been posted to the wrong question. It is not necessary to re-post it because you would be answering the same question not a different question. Your comment providing a link is good enough to re-direct users to a valuable answer. It will gain up-votes which will attract more attention. The link is also displayed more prominently to the right in the Linked column which serious users looking for good answers will be sure to check out.
Although the question where you posted your answer has been closed, so that further answers will be directed to the open question, your answer itself has not been deleted. The closed question and your answer will still appear in searches.