Find repeated substrings of length 3 in a string

It's quite fine. The logic is simple and clear. There might be a more clever solution (I don't know), but probably it will be less clear.

A few improvements would be good though:

• counter is set to 0 at two places, both unnecessary, as the value is overwritten later
• It's a good practice to declare variables in the smallest scope where they are used. For example counter and sub would be better to declare inside the loop.
• stringLen is not a great name, because it's not really the "length" of anything. Even limit would be better
• Instead of the magic number 3, and str.length() - 2, it would be better to make the target length a method parameter.
• Instead of calling .containsKey followed by get and put, I prefer to get, check null and then put, whenever possible. This is not always possible, for example when null is a meaningful entry, but that's not the case here.

Based on the above suggestions, the implementation could become a bit simpler:

public Map<String, Integer> findOccurences(String str, int length) {
    Map<String, Integer> map = new HashMap<>();
    int limit = str.length() - length + 1;
    for (int i = 0; i < limit; i++) {
        String sub = str.substring(i, i + length);
        Integer counter = map.get(sub);
        if (counter == null) {
            counter = 0;
        }
        map.put(sub, ++counter);
    }
    return map;
}

And it would be good to add a unit test for it:

@Test
public void test_This_This_() {
    Map<String, Integer> map = new HashMap<>();
    map.put("his", 2);
    map.put("Thi", 2);
    map.put(" Th", 1);
    map.put("s T", 1);
    map.put("is ", 2);
    assertEquals(map, findOccurences("This This ", 3));
}