Please try below code and let me know for any issues:
Looks like you are trying to get one textbox value at a time. You could use the GROUP BY clause in SQL to fetch your data.
SELECT
Country,
Region,
SUM([Sales1]) AS [First Sales],
SUM([Sales2]) AS [Sec Sales],
SUM([Sales3]) AS [Third Sales]
FROM
[dbo].[tblSales]
GROUP BY Country, Region
A single query will return all your results. You can take this results in a DataSet and then set the values of the corresponding textboxes by fetching values from the DataSet.
SqlDataAdapter adapter = new SqlDataAdapter(queryString, connection);
DataSet dataset = new DataSet();
adapter.Fill(dataset);
Now that you have records inside your DataSet, you can fill your textboxes.
DataRow dataRow = dataset.Tables[0].Select("Country = 'Italy' AND Region = 'North'").FirstOrDefault();
if (dataRow != null) {
textbox1.Text = dataRow["First Sales"].ToString();
}