If I read this correctly, you are using a group to blend the three data sources and you need to be able to update this group quickly.
It may be worth investigating using a small lookup table for all of your primary groups. You'd still have to manage them manually, but you wouldn't have to change anything in Tableau. This would be much easier.
Otherwise if a simple field calculation could serve as your grouping mechanism, built on dynamic criteria instead of literal strings.
Hope this helps.
Thank you Thomas,
A field calculation will not work since, the secondary sources are simply some pyramids that are being updated frequently. (Could be every day)
Concerning your first suggestion, I'm unclear on what to do...I can have a table to link the secondary source A with Primary Source and another table to link secondary source B with Primary Source that will be refreshed automatically then, but what next?
I'm still quite new with Tableau
Thanks a lot!