Thanks for the terrific question, Kathi! The only criticism I have is that your explanation only states why number 3 is wrong and does not discuss the difference between number 1 and number 2.
I was able to eliminate number 3 right away, because it would give you one result for each date, not for each year, but then I thought "Why in the world would you do SUM(SUM(TotalDue))? There must be something tricky about using an empty OVER() clause." After investigating that, I finally realized that SUM(TotalDue) / SUM(TotalDue) would just give you 1 for each row (assuming TotalDue is not 0 for a given year), so that isn't correct, and then I re-read the question and realized you were looking for the percentage of the grand total, in which case the first query was correct (the yearly total / the grand total).
Once again, thank you for making me think.