public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBox2.Text = textBox1.Text;
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            int spaces = 0, commas = 0;
            foreach (char ch in textBox1.Text)
            {
                if (ch == 32) spaces++;
                if (ch == 44) commas++;
            }
            if (spaces - commas == 3 && commas != 2)
            {
                string[] strMass = textBox1.Text.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
                string str = string.Join(" ", strMass) + ", ";
                textBox1.Text = str;
                textBox1.Select(textBox1.Text.Length, 0);
            }
        }
    }