Correct way to attach 3 layers of subfloor?
Given the amount you want to raise the floor I'd suggest 2x1&quot; planks screwed to each joist (+0.75&quot;) with the 3/4&quot; on top of that (+0.75&quot;). You should then put a sheet of 1/4&quot; ply on top. This way if you ever need to remove the tiles / ditra you only need to tear up the 1/4&quot; ply and can leave the 3/4&quot; floor intact.
Also, make sure you stagger the plywood joints.