When you are talking about 3W resistors, I just wonder if you are going to have a lot of low value resistors such as 1 ohm or even 0.1 ohm?
The thing is that cheap rotary switches can have significant contact resistance - I have often seen 20 to 50 milliohms. If you have 7 decades, that can be 350 milliohms of switch resistance alone.
A real quality switch can have 1 milliohm contact resistance or less, but they can also cost a lot of money. Hundreds of dollars for one switch is not unusual.
If you need accurate low value ohms, you may be better off with one box with 3 seperate chains of series resistors:
10 x 0.1
10 x 1
10 x 10
with a terminal for each resistor junction. These resistors can be 3W.
You could then probably make a decade box with much cheaper lower power resistors for doing values in the 100 ohm -100Mohm range.