If I had that, I wouldn't do anything but re-grease all the bearings and chain, being careful not to clean any crud off of it.
I would love to roll up to my friends on their shiny carbon bikes and sniff at them!

Of course you'll want to get the brakes working well, but maybe you can just replace the inner cable and leave the dirty outer.
It may not be a 3 speed, it may be that you loosen the wheel and shift the chain over, basically you get 3 ratios to choose from, but not while riding.

Pretty soon the spokes are going to destruct, but not all at once, just don't do any fast downhills.