I'm replacing an old ceiling light with chain with a light with outlet attachment in a grounded metal box. Do I need to ground the fixture since I have a 3 prong outlet, or since box is grounded I don't need to?
You should ground to the box, the light, and the outlet if they are metal and you are able, i.e. it has a ground wire or ground screw.
There is no harm in pigtailing off a short piece of wire to ground everything you can; it'd be worth it if anything were to ever short out.
And since I'm unsure by your post, I'll clarify this.. just because an outlet has 3 prongs does that mean that it is grounded. You still have to run a ground wire to it to make it grounded. Same concept as that the receptacle wouldn't be powered until a hot wire were ran to it.