Yikes--sounds like a concrete sandwich---
My question to you is --Do you want to do this floor right? Or just do it and see what happens?
Right would be to peal off the top two layers--leaving only the 3/4 sub floor-
add 1/2 inch ply on that--for a total of 1 1/4 " ply--then 1/4 inch cement board then tile.
I think the house was being prepped for tile(the cement board) and the buyer changed the plan to sheet goods.
Tile directly over plywood of any kind is a poor installation--wood moves-tile doesn't. If wood get wet it swells--tile doesn't do well if the surface is swollen.