It is hard to deal with all the oinfo given. Did your nephrologist say you had Class 3 for sure? I have Class 4, the most serious - I also know there is a class 5 that you would think is more serious, but can be mild to severe.
One thing I was thinking about is that there is so much more that goes into your kidney health than just class of nephritis;proteinuria, creatinine, etc. So, perhaps your nephrologist thinks that due to these other factors you are in good shape. Dictionary definitions give generalizations, but don't always have the total picture.
I think a second opinion is always a good idea. Even though I had to travel 60 miles to get one, my local nephrologist suggested it as well.