Hmm.. The X-Care warranty has been popular here but on model 3s in particular, there are not a ton of extended warranties. Its only been out since very late 2017 and been sold in volume starting in mid 2018. "most" people dont have an extended warranty on these, especially since there isnt any "finance office" to push people to sign up for it.
I believe they might be more popular on the S and X (which also have more parts) but you might want to check out those subforums to see.
As for salvage, my personal thought is its not worth it on a tesla. Not in the slightest. Since tesla has a policy of turning off any DC fast charging on a salvage vehicle (once they get around to it) that means that, eventually, any salvage car is going to lose fast charging. Unless one is just going to drive in their neighborhood, then Losing access to all fast charging on the road (not just teslas) is a show stopper, to me.
These are the most connected cars in the world, so, tesla will eventually "find it" so to speak, unless one is going to forgo pretty much all app access, root the car, etc etc.
There are people who buy these salvage, etc, but it isnt a road for the faint of heart, and there is always a lot of argument about "tesla cant do XXX". But tesla DOES do XXX (like disable all fast charging, for example).
Its not my demographic (salvage vehicles) anyway, but in my opinion, a salvage tesla model 3 is worth more for the battery possibly being used for some other application, than it is as a vehicle.