You can try below code
protected void LinkButton2_Click(object sender, EventArgs e)
{
foreach (GridViewRow row in GridView3.Rows)
{
var chk = row.FindControl("Chk") as CheckBox;
if (Request[chk.UniqueID] != null)
{
string bag = WebConfigurationManager.ConnectionStrings["AdminPanel.Properties.Settings.ConnStr"].ConnectionString;
SqlConnection con = new SqlConnection(bag);
SqlCommand cmd = new SqlCommand("icerik_Delete", con);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("icerikID", GridView3.SelectedRow.Cells[1].Text);
con.Open();
cmd.ExecuteNonQuery();
con.Close();
}
}
}