I recently install 3 GFI receptacles outside. They are wired correctly with ground attached in a 3 wire (copper) system. The boxes I used are outdoor plastic boxes. So the ground wire does no good being attached to this box. Why does it show open ground on my tester? Thank you

Either the tester is faulty, which I doubt, or the bare or green equipment ground wire is not connected proper someplace.

The ground wire must connect to each of the green screws on each outlet, all must be spliced together, so that there is a path back to the grounding or neutral bar back at the panelboard.

Be sure to note at the panel if there is only one bar for both neutral whites and green/bare grounds, or is there a separate bar just for the green/bare grounds. If the main breaker is in that panel, then white neutrals and grounds will be together. If there is a separate ground bar, then only white neutrals are on one bar, and green/bare grounds are separated on another bar.

If you extended these new outlets from an existing circuit, then there is a problem with the grounding of this circuit.

First of all, thank you very much for your quick reply. And I took your advice and checked the existing house circuit and found that the builder did not hook up the ground wire to a previous receptacle on this circuit. That does surprise me. When we bought the house. I changed the receptacles over from the ivory colored ones to white and found in each receptacle ground wires were not connected. I could see by the end of the wire they were never hooked up. I thought this was part of electrical inspections. Anyway, many thanks for your help!

If your builder took that serious of a shortcut I would check out everything else he did!