Hi, interesting question and indeed not a bad idea, since the databases fit somehow together methodologically and are to some extent complementary.
The world food database is somehow only available as SimaPro file though (maybe project lead Quantis was paid to make it only available in SimaPro format, or the companies wanted to limit its application - in any case not too nice I think); it can be imported into openLCA though when exported from SimaPro, with some few tricks, I tried this some time ago.
For a smoother integration of the three databases, you should have at least two of them in JSON-LD format, then you can specify what happens with elements already existing in the target database.
Best wishes,
Andreas