The tubes are for closer focusing than the lens normally allows, and the order in which you mount them doesn't really matter at all. When you put a tube in place, it moves the lens further away from the camera's sensor/film plane, so the lens will no longer focus at infinity. The more tubes you put in place, or the longer the tube (if you're using the variable tube), the closer the lens will focus, but the more limited the most-distant focus point becomes. (Mirror lenses usually focus slightly beyond infinity, so you will keep some focus leeway, even if you lose the ability to focus at infinity.)
The actual exposure compensation you'd need to apply depends on the exact design of the lens. Catadioptrics (mirror lenses) have a complicated folded light path that has the effect of the lens having the stated focal length, but that doesn't tell you what's going on at the back of the lens -- it could have a design that has the light path of a 500mm "simple" lens, or it could be wider (which allows greater focus variation with a shorter helix twist) or narrower (which makes focusing more precise, but much more limited). In any case, you won't be worrying about 2X or 3X. If your 2-3X variable tube extends 1cm when you go from "2X" to "3X", then you've extended the distance of a 50cm (500mm) lens away from the sensor by 3cm with just that tube in place and rotated to "3X". That's not enough exposure difference to worry about, but it likely means that you can focus on things that are about 6 feet away, which is closer than you'd normally be able to focus. If your subject is closer than that, add more tubes.
You may find that with all of the tubes in place, you'll need to add a third or a half stop of exposure if you're using an external meter. And remember that your lens may be f/8 in terms of the diameter of the front element, but that big circle in the middle (the back side of the front mirror) and the hole in the rear mirror means that it's not an f/8 in terms of exposure; it's more like an f/10 or f/11. The f/8 only tells you what your depth of field will be, not how much light makes it to the sensor. If you're using an external meter, you need to test to find out what the T-stop (the effective aperture when it comes to exposure) is.
If you're metering through the lens, it really doesn't matter; the compensation will be done automatically for you.
