Good news. That still doesn't mean that there couldn't be a ring problem but the only way to know for sure is to tear it down. I don't think you need to do that yet. What I would do at this point is replace #3 or all the seals if you like. When changing seals with the spring off give the valves a wiggle test to check the guide for excess wear. Put a fresh plug in #3 or all if you like. Drive it and see if #3 fouls again. Keep track of your mileage. How did the rest of the plugs look? Another thing you could do while the plugs are out is to visually inspect the cylinder walls. Maybe you know someone with a bore scope that could check it for you.