using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public int a, b, c;

        public Form1()
        {
            InitializeComponent();
        }

        private string JHandler(object sender, ref int value)
        {
            CheckBox checkBox = sender as CheckBox;
            int tag = int.Parse(checkBox.Tag as string);
            value = checkBox.Checked ? value + tag : value - tag;
            return value.ToString();
        }

        private void CheckBox_A_Click(object sender, EventArgs e)
        {
            textBox1.Text = JHandler(sender, ref a);
        }

        private void CheckBox_B_Click(object sender, EventArgs e)
        {
            textBox2.Text = JHandler(sender, ref b);
        }

        private void CheckBox_C_Click(object sender, EventArgs e)
        {
            textBox3.Text = JHandler(sender, ref c);
        }
    }
}