get random value from array with offset

There are 3 groups of words (0-2) with lengths:

0: 3-6 Low difficulty

1: 7-10 Medium difficulty

2: 11-14 High difficulty

Data array with words in order based on difficulty level:

[0,1,2,
 0,1,2,
 0,1,2,
 ...]

String[] wordsList = new String[]{
"Dog",    // 3
"Adventure",    // 9
"Pronunciation",    // 13
...}

How to randomly pick word from array based on difficulty level?

if (diff == 0) word = should be wordsList[0] or wordsList[3] or wordsList[6] ...

I tried this but it did not work:

int diff = 0;
String word = wordsList[new Random().nextInt(wordsList.length/3) + diff];