It depends on what you want to do with the data after that; if you just want the number, perform the following:
Make a calculated field in Excel. sum([Sales]) with a fixed [country]: TotalSalesPerCountry (calculates the total sales per country)
Create a calculated variable called CompanySalesPerCountry using the following formula: fixed [country]: sum
(IIF([Product] = 'A' OR [Product] = 'B' OR [Product] = 'C',[Sales],0)) (calculates total amount for each nation when [PRODUCT] = A, B, or C)
AVG([CompanySalesPerCountry]) / AVG([CompanySalesPerCountry]) / AVG([CompanySalesPerCountry]) / AVG([CompanySalesPerCount ([TotalSalesPerCountry])
Change the final field's Properties to 'Percent', then drag [Country] to the rows and [MarketShare] to the columns.