Hi Gabriel, do you have the option of combining all three data sources into one data source?

Yeah, that's probably your best bet. I don't think you can reference LOD in secondary data sources.

I think the main issue is that you can't use an INCLUDE or EXCLUDE LoD in a secondary source (because those depend on the view level of detail which is determined by the primary data source).  But you can use a FIXED LoD.

So, if Supplier and Check Number give you a unique check then you should be able to use a FIXED level of detail in the secondary:

{ FIXED [Supplier], [Check Number]:

    max([Check Amount])

}

One thing that might keep this from working is if you are using secondary source dimensions as filters.  Since you are creating a scaffold, you should be able to build a scaffold data source that allows for all filtering using only primary fields.

Hope that helps!

Joshua