Is it always "letter letter digit", or can the digit also appear elsewhere? If it is the former then it's quite easy, considering that letters are chars and chars can be treated as ints. Math.random() or java.util.Random can then help:
The digit can be generated similarly except without the last step: Use a StringBuilder to add these all together, then at the end call toString().
Now, if the digit can appear anywhere then it's a bit harder but I'm sure you'll be able to work it out using my example code.