As others have said , you only need a three way switch if 1) your inverter does not have a built in transfer switch
and 2) your inverter can handle all the loads (otherwise, you have the complication of turning off those loads it doesn't handle before switching)
Why not get one of these nice NewMar switches?
Newmar AC Ship Shore Switches-AC Newmar - Star Marine Depot
This reminds me of a year or two ago when I got some major internet flaming here for urging folks to have a separate inverter sub-panel. Ah well....