If it's been outdoors in Missouri for three years I wouldn't recommend getting your hopes up. *Everything* is likely to be badly oxidized, and if any standing water made it into the case there's probably a thick coating of organic slime on anything it touched. (This is the South we're talking about.)

Pretty much your only chance will be to take it completely apart, scrub it as clean as you can, and check the board for serious corrosion or pitting. If by some miracle it looks intact then you can try preemptively replacing every electrolytic and paper capacitor (they'll all be bad) on the board and firing it up, preferably with a new power supply. Seriously though, you're likely to end up chasing things like rusted shorts inside the expansion slot connectors and under the IC sockets... it's not going to be fun.