binding = new Binding("Text", cmb_PrefConNumber, "Text");
cmb_PrefConNumber.DataBindings.Add(binding);

if (!cmb_PrefConNumber.Items.Contains("Alternative"))
{
    cmb_PrefConNumber.Items.Add("Alternative");
    return;
}

private void textBox1_TextChanged(object sender, EventArgs e)
{
    if(!String.IsNullOrEmpty(textBox1.Text))
    {
        PopulateCombo(1);
    }
}

private void textBox2_TextChanged(object sender, EventArgs e)
{
    if(!String.IsNullOrEmpty(textBox1.Text))
    {
        PopulateCombo(2);
    }
}

private void textBox3_TextChanged(object sender, EventArgs e)
{
    if(!String.IsNullOrEmpty(textBox1.Text))
    {
        PopulateCombo(3);
    }
}

private void PopulateCombo(int textBoxID)
{
    //With this you will get how many textBoxes have value
    int filledTextboxes = 0;
    filledTextboxes = (String.IsNullOrEmpty(textBox1.Text)) ? filledTextboxes++ : filledTextboxes;
    filledTextboxes = (String.IsNullOrEmpty(textBox2.Text)) ? filledTextboxes++ : filledTextboxes;
    filledTextboxes = (String.IsNullOrEmpty(textBox3.Text)) ? filledTextboxes++ : filledTextboxes;

    //With this you will run one code if only one textbox has value and other if more than one has value
    if(filledTextboxes == 1)
    {
        switch(textBoxID)
        {
            case 1:
                //Populate comboBox with some value
                break;
            case 2:
                //Populate comboBox with some value
                break;
            case 3:
                //Populate comboBox with some value
                break;
        }
    }
    else
    {
        //What happends when there are multiple textbox with value
    }
}