To put these numbers in descending order, we need to convert them to a common format. 
- 1/4 = 0.25
- 0.4 = 0.4
- 21% = 0.21
- 0.34 = 0.34
- 3/100 = 0.03
Now, we can arrange them in descending order:
0.4, 0.34, 0.25, 0.21, 0.03
Therefore, the numbers in descending order are: 0.4, 0.34, 0.25, 0.21, 0.03.