Which one of these three configurations consumes the least resources?
Unity is based off of GNOME, and uses more RAM. Cinnamon is a little less resource hungry, but if you are looking for a desktop environment when resources is a factor, you might want to give LXDE a try. It's a fairly pretty desktop environment that runs in 128 megabytes of RAM. It has an interface very similar interface to KDE with less eye candy.