I have 3 different select queries querying 3 different tables in my DB.I need all of them to be executed at once as a batch and retrieve the results and fill a dataset with 3 different tables representing the results of 3 different select queries.How do i do that???

After executing the below mentioned code 'am getting only the first select queries data.

Code:
  
  #region QUERIES
        string getCustomers     = "SELECT * FROM Customers";
        string getCategories    = "SELECT * FROM Categories";
        string getEmployees     = "SELECT * FROM Employees";
        #endregion

        public DataSet GetData()
        {
            DataSet data                = new DataSet("Batch Data");
            OleDbConnection connection  = new OleDbConnection(Properties.Settings.Default.nwindConnectionString);
            OleDbCommand command        = connection.CreateCommand();            
            OleDbDataAdapter adapter    = new OleDbDataAdapter();

            adapter.SelectCommand = new OleDbCommand(getCustomers, connection);
            adapter.SelectCommand = new OleDbCommand(getCategories, connection);
            adapter.SelectCommand = new OleDbCommand(getEmployees, connection);
           
            try
            {
                connection.Open();
                adapter.Fill(data);
            }
            catch (OleDbException error)
            {
                System.Diagnostics.Debug.WriteLine(error.Message);
            }
            finally
            {
                command.Dispose();
                adapter.Dispose();
                connection.Dispose();
            }
            
            return data;
        }