In general it is not a good idea to cascade switches under load as the
switches may not switch at exactly the same time. For instance, the shore
power may be momentarily connected to the inverter. Since they are not in
phase each source could be at its peak voltage of opposite polarity from the
other. Larges currents will flow and usually the power station beats the
inverter every time and your switches could get damaged from the high
currents. I would recommend a switch that switches between the 3
simultaneously, you need a "three pole" switch (technically a 6 pole if you
want the neutrals isolated from one another). This item may be of interest:

http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/12645
5/10001/294/124/9

Hope it helps!

Dan,

Lots - This is what I do for a living, but mostly for boats (same-same).
(Would you be surprised if I told you that I get to rewire a few of
these every year?)

First a question:
Are you thinking of a manual or an automatic transfer?
Little matter - connection is the same.
< As I was writing this up - This suddenly got interesting.>
<If you have a Xantrex inverter-charger and 50amp three wire feed this
is going to get interesting.  In fact, I'm going to have to think about
this a few days.>

Next:
Most good (big) inverters have an automatic transfer built in.

If yours does not, you will need two transfer switches.
The first one selects between shore power and the generator and the
second selects between the output of the first and the inverter.

You you know if your unit is two wire or three wire?  (Is the main
breaker in the AC box one pole or two?)
Most 50amp RVs are actually three wire (this is actually 230v<115+115>).
      That is why that big assed plug has four pins
(HotL1-HotL2-Neutral-Ground)
Most generators are two wire.
Nearly all inverters are are two wire.

As all of your wiring is "isolated" you are going to want to switch the
neutral also.

Dan, I'm stalled here until I know more about your equipment.
Is the unit actually Three Wire.
Is the Generator Two or Three Wire?
Who made the inverter you are using?

We can figure this out.  There are more than a few blue water cruisers
out there with a couple or four 8D, an 8KVA diesel genset and a shore
power cable (but 50amp three wire is rare in the marinas).

Come back and tell me something.

Matt Colie