I have a garage with just the shell of a fuse box put up, but I now need some electricity, not a lot, in there. Should I finish the fuse box or just run an extension cord from the house with a 3 way plug-in?
Fuse box for sure! It is a lot more safe. Extension cords for the long haul could be dangerous. If you are putting in a fuse box then you must already have electric to the garage.
For safety reasons, I would finish the fuse box. Using an extension cord from the main house out isn't really all that safe. Besides you could blow a fuse in the house from overloading.
If you put circuits in the box, you can always just put in what you need. You don't have to fill it up. If one 10 amp circuit can service 5 regular outlets (which it does) and that's all you need, just put that in. You wouldn't even need a breaker bar that shuts the whole thing down.
But it sounds like you might want to read up on this. Electricity isn't a guessing game.