> Should I apply the treatment on top of the middle hive body or on
top of the bottom hive body.

Your heading says three deep and your post says three medium. If three
medium  (which is about like two deep bodies) I would do like below. I have
no experience with treating a three deep box with ApiLife Var so not sure
how to answer your question. Perhaps my friend Eric Brown (Brushy Mountain)
will answer?
My guess Eric would still be to place at the top of the box but only a guess
but what is done with menthol packets on a three deep box.

Break one tablet in four pieces and place on the four corners of the top box
under the lid. The bees do not need to contact the thymol to work. A strong
hive will try to remove the thymol or propolis over. You do not need to
cover the thymol with a screen as suggested early on by testing done in
Georgia by Delaplane.

Sincerely,
Bob Harrison

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