Check trivial values first.
works for 1 as well
Now assume that is divisible by 3 for all integer vaule of upto a particular value
I see you attempted to evaluate
So if is divisible by 3 so i as and are divisible by 3 then so is ad infinitum.
Therefore by induction is divisible by for all .
Edit: I fixed my solution it is correct now. But this isn't the best method. Galactus post is more useful.