Off hand, I can't explain the three lights. These tester have very simple logic circuits in them, which can be "confused" when they find a circumstance not in their "program". They are known to indicate an open neutral as "hot/ground reversed". I would not change anything until you find out the reason. If the manufacturer name is on the tester, you could try to find a phone number and call them.
Other wise, the next step would be for an electrician to verify the power, neutral, and ground connections. Unless your tester is just a lemon, the electrician can figure out what if anything is wrong on the circuit.