Sum every third number

Write a function sumOfThirds(arr), which takes an array arr as an argument. This function should return a sum of every third number in the array, starting from the first one.

Instructions

• If the input array is empty or contains less than 3 numbers then return 0.
• The input array will contain only numbers.

Example test cases

sumOfThirds([1, 1, 1, 2, 2, 2, 3, 3, 3]); // Output: 6
sumOfThirds([]); // Output: 0
sumOfThirds([-5, 0, 5, -4, 1, 6, -3, 2, 7]); // Output: -12

Hints

• You can use the built-in method .reduce().