NOT A TEACHER
(1) The teachers have given you excellent advice.
(2) May I add my two cents?
(a) Maybe native speakers are having a problem with those sentences because
such a question would never be asked in an English-speaking country. If three
women came to a meeting with a man and discovered that he was going to ask them
to explain why he should choose one of them to marry, all three ladies would
slap him in the face and walk out.
(b) There is one very little difference: The first sentence is said without a pause; the
second sentence with a pause (please notice the comma):
Among you three, whom should I marry?
(c) There is also another difference. If the three ladies do not know why you called
them to the meeting, then the second sentence is more dramatic and will make a
bigger impression on them. Why? Because the magic word "marry" is the very
last word in the question.