check 2d array diagonally?

I'm trying to search a 3x3 2d array diagonally, like this:

I want to check if all boxes in the diagonal have the same value. Here is how I try to do it:

thisOne = board[0][2];    //set to 'X'
    for(i = 0; i<3; i++) {
        for(j = 3; j>0; j--){
            if(board[i][j-1] != thisOne) {
                thisOne= '\0';
            }
        }
    }
//since all boxes were 'X', thisOne is still set to 'X'
if(thisOne != '\0') {
    winner = thisOne;
    printf("vinnare på nördöst\n");
}

So after running this code, winner should be 'X', if all boxes are X's. But the code does not do that, why is that?