Hi JRDavis,
A few things to think about.
3 coats of any "seasoning" is not a seasoned piece. It is more of a pre-seasoned piece, so you do not have the same protection.
Not sure of how you are storing your iron, basement, garage, V. displayed, hanging in the kitchen etc. the second part to this is where you live, high humidity, rainy, dry.
A lot of factors can play a part in your issue, but remember it still is iron and if the "seasoning" flakes, cracks, gets damp, it is going to get rust spots.
I might suggest that you season your iron with a few more coats with another oil, but it still iron. I am not going to say anything about what you are using for your seasoning because I know nothing about it. But I have found that using 2 different types of oil / fats during your seasoning makes a difference, at least for myself. I have never had any of your issues, yet.
You could try stripping one of your pieces and season it with another type of oil, just to see if you have an issue with your current choice.
A large part of my iron is hanging in my office, kitchen and basement. For most of the year my home is temp controlled, so very low humidity, yes even in my basement. The other thing is all my iron gets at least 5 coats of oil / seasoning before it goes into my home for display or use, just me liking the color after and the extra protection. Fact 99.5% of my iron is displayed and not used, eye candy.
Maybe someone else will offer some other ideas.