When finding out which number is the greatest, I would convert these numbers to numerals with decimals for easier comparison.
3 is approximately 6.71.
6 is 6.25
6.55 is, well 6.55.
From here, we can tell that 6.71 is the greatest, followed by 6.55 and 6.25 is the smallest. Thus, the order in descending order should be 3, 6.55 then 6, which is Option A.