item1ToolStripMenuItem.Click += new System.EventHandler(ToolStripMenuItem_Click);
item2ToolStripMenuItem.Click += new System.EventHandler(ToolStripMenuItem_Click);
item3ToolStripMenuItem.Click += new System.EventHandler(ToolStripMenuItem_Click);

private void ToolStripMenuItem_Click(object sender, EventArgs e)
{
  comboBox1.SelectedIndex = comboBox1.FindString(((ToolStripMenuItem)sender).Text);
}