My 2 cents...
1. It gets cold.
2. Not as much traffic in and out.
I was told 3 full frames, but supplementing with sugar syrup is advised. This should be done while they are still active. I have left them with more than 3 frames as I want to go down the 'natural' path which avoids using sugar syrup. (I will make the exception if bees I have collected would struggle to survive without the assistance)
Some keeps recommend:
1. Decrease the size of the opening.
2. Put match sticks under the lid for ventilation. (concerns over humidity and mould)
3. Drop the height of the hive stack to allow the bees to manage the temperature of the hive easier.
4. If you have a queen excluder, you must remove it or you won't have a queen come spring.
5. Kiss them goodnight.
Sure to be more, but that's a start.