My question to you, Kenny, is "why?" The way I'd do it is to have a regular power switch and use a closed-circuit DC input jack. The battery current runs through the jack to the amp when there's nothing plugged into the jack, and the amp runs from the wall when you plug something in that way.
Regarding the other suggestions, the SPDT on-off-on one is the only one that will work. I don't think you'll find a 3P3T switch, and if you did it would be either very big or very expensive. And the DPDT idea doesn't work at all if you use a standard on-on type: there is no off position. You could use a DPDT on-off-on switch, but the only difference between that and stereth's SPDT idea is that you're switching both legs of the power, which is nice but not essential.