problem in combo box select value?

The requirement sounds a bit odd since if all of the combo boxes follow the selection of one combo box, what is the purpose of the other combo boxes....

Anyhow you didn't mention what UI technology you use but if this is for example Forms, you would wire SelectedIndexChanged[^] event and in that event handler you would set the SelectedIndex[^] property for the other combo boxes.

For example

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) {
   this.comboBox2.SelectedIndex = this.comboBox1.SelectedIndex;
}

The basic idea is the same also with other UI technologies.