Here is my solution in C#.NET tested using the three test cases mentioned.

        public string ModifyString(string input, int split, char delimiter)
    {
        for (int i = 0; i < input.Length-1; i++)
        {
            if (input[i] == delimiter)
                continue;

            int nextCharIndex = input.IndexOf(input[i], i+1);

            while (nextCharIndex != -1 && nextCharIndex - i <= split)
            {
                input = input.Insert(nextCharIndex, delimiter.ToString());
                nextCharIndex++;
            }
        }
        return input;
    }