16821 ÷ 3 = 5607 R0
We're going to perform division on 16821 with 3 as the divisor.

Step 1:
3 goes into 1 0 times with a remainder of 1.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 1 to get 1.
Bring next digit (6) of the dividend behind the 1 and repeat the process: 16 / 3

Step 2:
3 goes into 16 5 times with a remainder of 1.
Write down 5 as next digit of of the result. 
Result so far: 5
Subtract 15 from 16 to get 1.
Bring next digit (8) of the dividend behind the 1 and repeat the process: 18 / 3

Step 3:
3 goes into 18 6 times with a remainder of 0.
Write down 6 as next digit of of the result. 
Result so far: 56
Subtract 18 from 18 to get 0.
Bring next digit (2) of the dividend behind the 0 and repeat the process: 2 / 3

Step 4:
3 goes into 2 0 times with a remainder of 2.
Write down 0 as next digit of of the result. 
Result so far: 560
Subtract 0 from 2 to get 2.
Bring next digit (1) of the dividend behind the 2 and repeat the process: 21 / 3

Step 5:
3 goes into 21 7 times with a remainder of 0.
Write down 7 as next digit of of the result. 
Result so far: 5607
Subtract 21 from 21 to get 0.

The final result is 5607 with a remainder of 0.