Existing username is not working for some usernames, why?

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ASP.NET

C#3.5

Hell to all,

I have a web app that checks to see if a user exist within three tables. If the user exists in table2 and table3 then the user can create a username and password. If the user exists in table1 then the user can't create a username and password. So far this works on some of the user accounts we have tested but on others, we can create double usernames with different passwords. Please let me know where I am wrong at. Is there a way to do it better?

protected void Page_Load(object sender, EventArgs e)
        {
            if (IsPostBack)
            {
                SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["C3POConnectionString"].ConnectionString);
                con.Open();

                string cmdStr = "Select count(*) from Table1 where EmailAddress='" + TextBoxEA.Text + "'";
                string cmdStr2 = "Select count(*) from Table2 where EmailAddress='" + TextBoxEA.Text + "'";
                string cmdStr3 = "Select count(*) from Table3 where EmailAddress='" + TextBoxEA.Text + "'";

                SqlCommand userExist = new SqlCommand(cmdStr, con);
                SqlCommand userExist2 = new SqlCommand(cmdStr2, con);
                SqlCommand userExist3 = new SqlCommand(cmdStr3, con);

                SqlCommand cmd = new SqlCommand("select USERID, EmailAddress from Table1", con);
                SqlCommand cmd2 = new SqlCommand("select USERID, EmailAddress from Table2", con);
                SqlCommand cmd3 = new SqlCommand("select USERID, EmailAddress from Table3", con);

                userExist.Parameters.AddWithValue("@EmailAddress", TextBoxEA.Text);
                userExist.Parameters.AddWithValue("@Password", TextBoxPW.Text);
                userExist2.Parameters.AddWithValue("@EmailAddress", TextBoxEA.Text);
                userExist2.Parameters.AddWithValue("@Password", TextBoxPW.Text);
                userExist3.Parameters.AddWithValue("@EmailAddress", TextBoxEA.Text);
                userExist3.Parameters.AddWithValue("@Password", TextBoxPW.Text);

                int temp = Convert.ToInt32(userExist.ExecuteScalar().ToString());
                int temp2 = Convert.ToInt32(userExist2.ExecuteScalar().ToString());
                int temp3 = Convert.ToInt32(userExist3.ExecuteScalar().ToString());

                <pre>if (temp == 1)
                {
                    ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('Sorry, This User and Passsword Already Exists');", true);
                    TextBoxEA.Text = string.Empty;
                    TextBoxINST_ID.Text = string.Empty;
                    TextBoxaccessLevel.Text = string.Empty;
                    TextBoxEA.Focus();
                }
                else if (temp2 == 1 && temp3 == 1)
                {

                }
                else if (temp2 == 0 || temp3 == 0) 
                {
                    ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('User Name is Not Recognized by the System. Your Accreditation Liaison Officer (ALO) has permissions to this page. Please contact your ALO.');", true);
                    TextBoxEA.Text = string.Empty;
                    TextBoxEA.Focus();
                }
con.Close();
            }
        }

What I have tried:

I have tried to add code to the temp2 and temp3 and they work from time to time.