You can get the definition of this idiom here.
I think that the choices no. 2 & 3 are similar in meaning
In the UK we often say "a drop in the ocean" for the same thing.
As I never use the 'bucket' variant, I assumed it must have a different meaning from the 'ocean' one. Just as well I never use it
The thread title does not match the idiom. The expression is "a drop in the bucket".