New light fixture - grounding wire connection
Generally speaking, all grounds can be bundled. You want the fixture, any metal boxes and conduit, and any switches all connected to the grounding conductor. 

In your case, bundle or chain them all together in the simplest and most secure manner available. Don't put more than one wire under a screw, and use appropriately-sized nuts where needed. 

Use short lengths of wire to make jumps from screws to nuts, or from nuts to other nuts. As element11 said in a comment, 


  This is what is known as pigtailing. If you have multiple grounding
  wires, take a short 4-6" piece of ground wire and connect it to the
  screw terminal in the box. Then use a wire nut to connect this wire to
  all other grounds that enter the box, such as the fixture ground and
  the ground wire going back to the panel.