if(RadioButton1.Checked)
   Label.Text = first
else if(RadioButton2.Checked)
   Label1.Text = second
else if(RadioButton3.Checked)
   Label1.Text = third
else 
   ...

------解决方案--------------------
将3个RadioButton的CheckedChanged都绑定到下面的radioButton_CheckedChanged事件上。
C# code

        private void radioButton_CheckedChanged(object sender, EventArgs e)
        {
            RadioButton rb = sender as RadioButton;
            if (rb.Text == "radioButton1")
                this.label1.Text = "first";
            else if (rb.Text == "radioButton2")
                this.label1.Text = "secord";
            else if (rb.Text == "radioButton3")
                this.label1.Text = "three";

        }

------解决方案--------------------
同意LS和LSS的  我一般将3个RadioButton的click事件调用同一方法   方法里面同LSS
  if(RadioButton1.Checked)
  Label.Text = first
else if(RadioButton2.Checked)
  Label1.Text = second
else if(RadioButton3.Checked)
  Label1.Text = third


------解决方案--------------------
        private void radioButton1_CheckedChanged(object sender, EventArgs e)
       {
           if (radioButton1.Checked)
           {
               label1.Text = "first";
           }
       }

       private void radioButton2_CheckedChanged(object sender, EventArgs e)
       {
           if (radioButton2.Checked)
           {
               label1.Text = "second";
           }
       }

------解决方案--------------------
C# code

        private void radioButton1_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton1.Checked)
            {
                label1.Text = "first";
            }
        }

        private void radioButton2_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton2.Checked)
            {
                label1.Text = "second";
            }
        }

------解决方案--------------------
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C# code

if(RadioButton1.Checked)
   Label.Text = first
else if(RadioButton2.Checked)
   Label1.Text = second
else if(RadioButton3.Checked)
   Label1.Text = third
else 
   ...


------解决方案--------------------
 private void radioButton1_CheckedChanged(object sender, EventArgs e)
       {
           if (radioButton1.Checked)
           {
               label1.Text = "first";
           }
       }