//1.在Form2中定义一个事件：
         public event EventHandler OnMybuttonClik;
//2.在Form2的按钮事件中触发它：
        private void button1_Click(object sender, EventArgs e)
        {
            if (OnMybuttonClik != null)
            {
                OnMybuttonClik(sender,e);
            }
        }
//3.在Form3中定义一个方法：
        public void Mybutton_Click(object sender, EventArgs e)
       {
            string str= this.textBox1.Text;
            int num = int.Parse(str);
            ++num;
            this.textBox1.Text = num.ToString();
        }
//4.在Form1中这样显示Form2和Form3:
        private void Form1_Load(object sender, EventArgs e)        
        {
            Form2 form2 = new Form2();
            Form3 form3 = new Form3();
            form2.OnMybuttonClik += new EventHandler(form3.Mybutton_Click);
            form3.Show();
            form2.Show();
        }

------解决方案--------------------
楼上正解啊，用委托最方便了，接分了