I would use just one event handler for all three checkboxes (btw, you know radio buttons would fit better here, don't you?) and validate using
XOR
function.
This way:
private void CheckBox_CheckedChanged(object sender, EventArgs e) {
CheckBox box = (CheckBox)sender;
if (box != null) {
if (box.Checked) {
if (box == toc_Blockleave) {
toc_Swap.Checked = false;
toc_Unavailability.Checked = false;
}
else if (box == toc_Swap) {
toc_Blockleave.Checked = false;
toc_Unavailability.Checked = false;
}
else if (box == toc_Unavailability) {
toc_Blockleave.Checked = false;
toc_Swap.Checked = false;
}
}
}
btn_Submit.Enabled = toc_Blockleave.Checked ^ toc_Swap.Checked ^ toc_Unavailability.Checked;
}
Hope this helps.