Easy to read JAVA solution:

public static int hourglassSum(List<List<Integer>> arr) {

    int arrLength = 6;
    int subArrLength = 6;
    int hourGlassHeight = 3;
    int maxSum = Integer.MIN_VALUE;
    int lastTopRow = arrLength - hourGlassHeight;

    for (int i = 0; i <= lastTopRow; i++) {
        for (int topCenter = 1; topCenter < subArrLength - 1; topCenter++) {
            List<Integer> topRow = arr.get(i);
            List<Integer> midRow = arr.get(i + 1);
            List<Integer> botRow = arr.get(i + 2);

            int topRowSum = (
                topRow.get(topCenter - 1) +
                topRow.get(topCenter) +
                topRow.get(topCenter + 1)
            );
            int midRowSum = midRow.get(topCenter);
            int botRowSum = (
                botRow.get(topCenter - 1) +
                botRow.get(topCenter) +
                botRow.get(topCenter + 1)
            );
            int currentSum = topRowSum + midRowSum + botRowSum;

            if (currentSum > maxSum) maxSum = currentSum;
        }
    }
    return maxSum;
}