filtering duplicate data across 2 tables

Hi,

I have looked for a solution elsewhere on this forum to my specific problem and cannot find anything that helps, but apologies if anybody knows of a posted solution and please point me in the right direction :)

The problem:

I am working with 3 data tables for this particular requirement. All 3 contain a field called 'store-ID'. The first (and biggest) table contains current year sales data and totals around 750k lines for 2016 YTD. The second table contains a list of stores that have not made any sales of a product in the past week, the third table contains a list of stores that have not sold our product in the past 4 weeks. Tables 2 and 3 contain only unique store ID's, but table 1 contains many repetitions of store ID.

I do not have a definitive list of all store ID's, but this is what I need to solve. The issue is that, although the sales data table will give me most of the stores, there are some that we know have never made a sale, so I need to combine the IDs from table 1 and 2, then remove all duplicates so that I can post a 'Total Stores' metric. I can work this in Excel, but I am struggling with DAX.

Any suggestions would be extremely welcome.