All other light combinations are described but no description for when all three lights are on. I am testing a circuit that was wired when I moved into the house. It is providing power and appears to be working however when I test other circuits in the house the lights display just the two Yellow lights indicating "Correct" (according to the key printed on the tester).
Thanks in advance for your help!

What does it mean when Sperry GFI 302A tester that has all three lights illuminated?

I see what you mean I read the instructions and it does not show a condition with all lights on.

http://www.sperryinstruments.com/documents/products/gfi-302a.pdf

You either have a defective wiring issue not addressed by the tester, or the tester is defective.

You can contact Sperry here for support:

Sperry Instruments (http://www.sperryinstruments.com/navi/customer-service)