Thanks! If a 2018 has the EAP upgrade, will it not receive the AP3 computer for free? I feel like there was announcement about that at one point but I could be wrong.
No, it wont get free new computer if you have EAP, it will only get free computer if you purchased Full Self Driving (which required the purchase of EAP as well).
Before tesla changed the packages and got rid of the Enhanced autopilot (EAP) when purchasing a model 3 new, you could get EAP without buying FSD.. and that version of EAP contains every feature the current AP + FSD cars do.
Anyway, the car would have to have the full self driving option purchased to get the new HW3 upgrade, and that upgrade will happen next year sometime.
As for why the OP is looking to buy used, it would be the same reason everyone wants to buy used... he / she wants to buy a model 3 cheaper (wants a deal). Nothing wrong with that, but this website seems to be the exact wrong place for those type of purchases. When I look at the "tesla for sale" pricing here, people selling used teslas in general want pricing that works out to be close to what the cars would be new.
They put things like ceramic coatings, and clear bra's and other enhancements on their cars, and think that somehow raises the price of their car (they want to try to recoup their investment in these things, but they usually dont raise your cars value).
(this is not unique to tesla... on bmw forums / magazines, people selling used BMWs bank on the fact they are an enthusiast selling to other BMW enthusiasts, and typically ask for more than market value for their car based on "always garaged, never had automatic wash,etc etc etc"
OP looking anyplace other than here would likely be better... and people selling model 3s still are trying to recoup as much as they can since their cars are only a year old or less... and they are selling privately trying to not take as much of a hit. You definitely want to look at the price of new model 3s, because since the price has changed a few time, its unlikely you will save a "ton" of money buying a used model 3. That likely needs a year or two more of inventory to balance out.