Colony shrank from three boxes to one in a month — normal fall dwindling or something wrong?

İzmir, second year, two hives. In late August my big colony was filling three boxes — bees covering every frame, a beard on the front every evening. Yesterday they fit in one box. The brood is down to three frames, the top two boxes have honey and hardly any bees, and I found the queen, walking around calmly, laying but not much. Everyone told me colonies shrink in autumn, but by two thirds in a month? I keep reading about mites and I'm scared. I did an alcohol wash in July and got 3 per 300 and treated with Apiguard in August. No deformed wings that I can see, no dead bees piled up, no fighting at the entrance. It's still 28°C here in the day. Is this normal for the Aegean or should I be doing something? Should I take off the empty boxes?