function sumArrayLinear(A, start, end){
  end = (end >= A.length ? A.length - 1: end); // avoid Out of Bounds
  var total = 0;
  for(start; start <= end; start++)
      total += A[start];
  return total;
}

var arr = [2,5,7,4,2,4,3,3,2,1,8,6,9,7,5,4,2,5,6,7,9,0,8,5,3,2,4,5,6,7,8,5,6],
    arrSum = [];

for(var i = 0, l = arr.length; i < l; i += 3) // loops 11 times (33 / 3)
  arrSum.push(sumArrayLinear(arr, i, i + 2)); // inner loop 3 times
                                              // Total: 11 x 3 = 33 operations
console.log("arrSum: "+arrSum);               // arrSum: 14,10,8,15,21,11,22,13,9,18,19