I got it how to do this. . .! Just set other two link button backcolor equal to transparent. Check below.
protected void Link_Click(object sender, EventArgs e)
{
LinkButton lbtn = (LinkButton)sender;
lbtn.BackColor = System.Drawing.Color.Blue;
string id = lbtn.ID;
if (id == "lnkcot")
{
// set other two to transparent
lnktex.BackColor = Color.Transparent;
lnkgar.BackColor = Color.Transparent;
}
else if (id == "lnktex")
{
lnkcot.BackColor = Color.Transparent;
lnkgar.BackColor = Color.Transparent;
}
else if (id == "lnkgar")
{
lnkcot.BackColor = Color.Transparent;
lnktex.BackColor = Color.Transparent;
}
}