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

In a string s of lowercase letters, these letters form consecutive groups of the same character.
For example, a string like s = "abbxxxxzyy" has the groups "a", "bb", "xxxx", "z", and "yy".
A group is identified by an interval [start, end], where start and end denote the start and end indices (inclusive) of the group. In the above example, "xxxx" has the interval [3,6].
A group is considered large if it has 3 or more characters.
Return the intervals of every large group sorted in increasing order by start index.
 
Please complete the following python code precisely:
```python
class Solution:
    def largeGroupPositions(self, s: str) -> List[List[int]]:
```