Answer:
2020
Explanation:
The rule that this sequence follows is that each number is the sum of the last three numbers:
1+1+1=3
1+1+3=5
1+3+5=9
3+5+9=17
5+9+17=31
We can see that every number is going to be odd because they are always the sum of three odd numbers, and three odd numbers always add up to be another odd number (odd+odd+odd=even+odd=odd).
Therefore, if all the numbers in the sequence are odd, all the numbers before the 2021st number will be odd, so our answer is 2020.