[Konstantinos]

pawn Код:

RandomEx(min, max) 
{ 
    return random(max - min) + min; 
}

pawn Код:

format(PlayerVehicleInfo[playerid][v][pvPlate], 32, "%c%c%c-%i", RandomEx('A', 'Z' + 1), RandomEx('A', 'Z' + 1), RandomEx('A', 'Z' + 1), RandomEx(100, 1000));

If that format will be permanent, you may want to change the size of pvPlate to 8.