I doubt it'll be 3x. Besides, customers likely paid less for the Air to begin with so probably not much difference in depreciation in absolute dollar terms.This will kill the resell value of the air 3 down to used budget ipad.... while the 2018 11 pro should keep a pretty high resell value... probably 2-3 times the air 3...
I got my 256GB LTE Air 3 for $630 brand new. Similar config on 2018 Pro 11 was $950-1000 discounted.
In any case, I don't sell my iPads. I just tend to hand them down to family. The Air 3, I'm likely keeping forever (last natively supported resolution on my favorite abandonware ereader app).