[thiaZ_]

You could try:

pawn Code:

stock getRandomLetter() {
    return 65 + random(52); // ASCII Code of capital A is 65 + 2 * 26 for all other chars in alphabet.
}

//...
format(someString, 128, "%c, %c, %c", getRandomLetter(), getRandomLetter(), getRandomLetter());

I'm not sure how number plates are shown, if there are capital letters only you could change the max. random value to 26 again.