2 and 3 are very similar if not identical. That doesn't mean these two phrasal verbs are interchangeable. In different contexts one or the other may be more appropriate. 'Fall off' tends to be more gentle. "Last year, sales fell off by 5%, but it was not nearly as bad as 2003 - when they dropped off by nearly 50%. [It was as is they'd dropped off a cliff.]" (You'll note that the increased violence of the change is reflect by the second use of 'dropped off' - where it is not a phrasal verb at all but a prepositional one).
The only way to distinguish them is to read as much as you can, and consult dictionaries - several (as you would consult a doctor - always keeping in mind that other opinions need to be considered).
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