Further, please TellTheDetails. That is, instead of simply saying 'it just wont work', please mention what is the problem - compile time error, runtime exception, or something else?
Now, coming to your problem:
Firstly, there are no 2D arrays in Java. There are arrays, and arrays of arrays, and arrays of arrays of arrays and so on.
So, what you are doing is - declaring an array of length 2. That will contain array of length 3 each. That will contain objects of class Arrays.
Now, why do you need Arrays object? It is used to perform operations on arrays (like binary search, sort, comparison etc.). You are not doing any of them. Further, all the Arrays specific methods are static, so it doesn't make any sense to create object of Arrays. But since we don't need Arrays itself, we'll not get into that.
Secondly, you are simply generating a random number and that's it. You are not assigning it to anything. One statements of your trial ones is correct (arr[i][b] = random.nextInt();). But to make it work, arr must be of type int instead of Arrays (just think about it - you cannot assign int to Arrays).