Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given a 0-indexed, strictly increasing integer array nums and a positive integer diff. A triplet (i, j, k) is an arithmetic triplet if the following conditions are met:

i < j < k,
nums[j] - nums[i] == diff, and
nums[k] - nums[j] == diff.

Return the number of unique arithmetic triplets.
 
Please complete the following python code precisely:
```python
class Solution:
    def arithmeticTriplets(self, nums: List[int], diff: int) -> int:
```