HI Try like this,,
string userName = txtUserName.Text.Trim();
string password = txtPassword.Text.Trim();
if (userName == "aaa" && password == "123")
Response.Redirect("MenuPage1.aspx");
else if (userName == "bbb" && password == "456")
Response.Redirect("MenuPage1.aspx");
else if (userName == "ccc" && password == "789")
Response.Redirect("MenuPage1.aspx");