3-rail, traditionally, doesn't seem to see much weathering that I've seen.
Weathering seems to be a factor only if you're looking to represent reality on your layout. In short, if you're just about 'playing with trains' or a collector, you have little need for it.
For finescale or people looking to represent reality as tightly as they can, it's essential.
You can, however, go too far. A pal of mine calls that "going full Malcom Furlow" in regard to the guy who used to write for Model Railroader and weathered stuff pretty harshly.
Here's what I have done, as I judge each car and structure on my layout individually in regard to what makes the most sense for its use in real life:
You can see that some stuff is dirtier than other stuff, in regard to how it'd been used or maintained...
