Random characters not using certain characters

I'm trying to create something that will be like a basic random password generator including uppercase, lowercase and digits - but for some reason this

        static void Main(string[] args)
    {
        bool validLength = false; 
        int userDefinedLength = 0;
        Console.WriteLine("How many characters would you like your password to be?");
        do 
        {
            try
            {
                userDefinedLength = int.Parse(Console.ReadLine());
                validLength = true;
                if (userDefinedLength < 3)
                {
                    Console.WriteLine("Please enter something larger than 3.");
                    validLength = false; 
                }
            }
            catch (Exception)
            {
                Console.WriteLine("Please input a valid integer length.");
            }
        } while (validLength == false);
        char[] passwordArray = new char[userDefinedLength]; 
        int asciiValue = 0;
        char asciiChar = ' ';
        bool validPassword = false; 
        Random ranAsciiGroup = new Random();
        Random ascValue = new Random(); 
        do
        {
            for (int i = 0; i < passwordArray.Length; i++) 
            {
                int randomAsc = 0;
                randomAsc = ranAsciiGroup.Next(1, 4);
                //Console.WriteLine(randomAsc);
                if (randomAsc == 1)
                {
                    asciiValue = ascValue.Next(65, 91);
                    //Console.WriteLine(asciiValue);
                }
                else if (randomAsc == 2) 
                {
                    asciiValue = ascValue.Next(97, 123);
                    //Console.WriteLine(asciiValue);
                }
                else if (randomAsc == 3) 
                {
                    asciiValue = ascValue.Next(48, 58);
                    //Console.WriteLine(asciiValue);
                }
                asciiChar = (char)asciiValue; 
                passwordArray[i] = asciiChar; 
                //validPassword = true;
            }


            bool isDigit = false; 
            bool isUpper = false;
            bool isLower = false;

            for (int i = 0; i < passwordArray.Length; i++) 
            {
                if (char.IsDigit(passwordArray[i]))
                {
                    isDigit = true; 
                }

                if (char.IsUpper(passwordArray[i]))
                {
                    isUpper = true; 
                }

                if (char.IsLower(passwordArray[i]))
                {
                    isLower = true; 
                }
            }

            if (isDigit == true && isUpper == true && isLower == true) 
            {
                validPassword = true; 
            }


        } while (validPassword == false); 

        Console.WriteLine("Your password is..."); 
        Console.ForegroundColor = ConsoleColor.DarkGreen;
        foreach (char c in passwordArray)
        {
            Console.Write(c);
        }
        Console.ReadLine();
    }

The password that it produces seems to not be using any numbers that are less than 6. And some of the characters that it produces are quite repeated - e.g. the lower case tend to have characters that appear much more than some others - or some that don't appear at all. I'll leave a 100 character example here.

m9nj88m8GBpF7Hk87E8p9CAE987pEj7pm7j89iHplo7DIpB87B9irlAk9Ik9C8q8i97B9o8l8GDjj88j898Dmk9A69969Ino988I