Hey Citation2,
As you point out, some things are constant variables - in this case - you (almost) always get a few drops more than expected.
Assuming that is (almost) always the case regardless of whether you do a flap full or flap 3 landing, then a flap 3 landing still saves 5kg-15kg (or whatever) of fuel compared to a config full.
To answer your question, assuming the fueller sets 6.0 in the fuel panel and you end up with 6080kg irrespective of your starting fuel, the fuel saving is where the refueller only uplifts 3075kg instead of 3080kg.