The issue is we have 3 databases that contain the names of our suppliers we also have data on those 3 databases that we want to merge into 1 pivot table. In order to do this, we need to get a list of unique items in order to do a many-many relationship match as shown in this post:
I would prefer to work in the Query Editor using the Append feature to combine the data from the 3 databases into one table. All your data modeling and visualisation tasks will be simpler from then on, for example you wont need to build this table.
However if you just want a quick fix using DAX, it would be along these lines:
All Customers = UNION ( UNION ( DISTINCT ( 'Invoice1'[CustomerID] ), DISTINCT ( 'Invoice2'[CustomerID] ) ), DISTINCT ( 'Invoice3'[CustomerID] ) )