Our ceiling has three wires...one white, one black, and one green. The new light fixture we bought only has a white and one black wire. What should I do with the green wire in the ceiling?
The green is the ground wire. If the work box is metal then the box itself should have a grounding screw and you should connect the green to that. Otherwise you don't need to do anything with the green wire.
Some fixtures dont have a ground wire - but do have a ground screw where you are supposed to connect to. The installation manual should tell you if that is the case.