I know how to do this with CSS, so in my mind the logic should be the same. However I cannot work out the logic using remainder in js

Here is how I need to split things:

1 file 'input.mp4' | 1 % 3 = 1
2 inpoint 1 | ?
3 outpoint 2 | 3 % 3 = 0
4 file 'input.mp4' | 4 % 3 = 1
5 inpoint 3 | ?
6 outpoint 4 | 6 % 3 = 0 
7 file 'input.mp4' | 7 % 3 = 1
8 inpoint 5 | ?
9 outpoint 6 | 9 % 3 = 0

So the issue I am having is, how can I target the 2,5,8. I cannot divide it by 2 as that will clash with any whole numbers. 4,6,8,10 etc