# Sum every third number

Easy

10

65.2% Acceptance

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()`

.