In order to estimate the size of the crowd, let's find the area of the parade route.
The length is 3 miles, that is, 3 * 5280 = 15840 ft.
The width is 2 times 21, so 42 ft.
Therefore the area is:
Now, to estimate the size of the crowd, let's use a rule of three, knowing that the area for 17 people is 7 * 7 = 49 ft²:
Rounding to the nearest integer, we have 230,881 people.