It depends on what is being kept in good condition. I think that:
- maintenance works better for structural or mechanical things, such as buildings and automobiles,
- upkeep works better for things that grow, such as hair and landscaping projects, and
- sustainance is more fitting for nourishment, or for something abstract, such as government.
Thus, at the legislative building, the democracy is sustained, the furnace is maintained, the front lawn requires upkeep.
These differences are mere generalities, though; the words are synonymous enough that the dividing lines I've indicated are neither hard nor strict.