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Which brand do you have?
Ido not know which one you have, but most do not light all 3 at the same time in a normal situation of any one fault.
I just checked my brands which are Ideal and GB and they are the same.
Now it could be, it is showing 2 faults at the same time, lets look at that possibility.
If you had an Open Ground, and at the same time a Hot/Neutral reversed, that may light up all 3 lights. Open ground is center light, Hot/Neutral Reversed is outside lights on the GB. Or an Open neutral and hot/ground reversed shows the same.
Now the Ideal, looks like is similar on the light responses.
So, it all depends on your make and model as to the lights, but is does seem it is outputting 2 faults at the same time since there isnt a single fault to light all 3 at the same time
If your label is still intact on the tester, look and see which 2 faults would represent the 3 lights simultaneously and then eliminate one of them to see the change. That will prove the situation.
Just rare for 2 faults
I think I will do a study of that in a bit on a test setup and see
I just performed a quick test on my workbench. Only took a minute. I wired for NO ground and Hot/Neutral reversed. What I got was Open Ground Light was bright and the Hot/Neutral Reversed lights were dim on my 2 testers. It appears they all lit, but you could easily see the difference in the illumination.