I was trying to do it the past 2 hours, and got a solution, but I'm not very convinced about it.
Here is what I did :
1) We have H(0) = 3 which is divisable by 3
2) We have H(1) = -3 which is divisable by 3, too
3) Let's have

which is the equivalance relation

- divisable by 3

- divisable by 3

- divisable by 3
So H(2) is divisable by 3 too, and since we have a recurrence relation, this applies to all positive integers following the relation.