I think all the important information has come out in prior posts.
@XPEL's is particularly informative regarding coverage and pricing.
My personal experience is with a 2013 P85+. When I first received the car, I immediately had it done in Opticoat family products. Opticoat, Optiglass, Optiwhatever. Maybe $800 to do the whole car, done by a guy who was fanatical about first cleaning and "correcting" the paint. Car looked fantastic.
Then I had a partial wrap done -- Xpel, covering the front hood and doors. I did the wrap *over* the Opticoat, obviously. Back then, some people recommended doing it that way, others recommended applying the Opticoat over the Xpel. I think the recommendation today is to do it on top of the Xpel. But to me, it made more sense then (and still does now) to put the Opticoat onto the actual base panels first.
I quickly found that (a) the Xpel was a "life saver" and (b) I hadn't wrapped enough of the car. Cost me $1700 to fix a small scratch on a rear panel. Worse, it cost about a week of actual car down-time at the auto body shop... on top of a several week delay to get into the auto body shop in the first place.
So I went back to the Xpel guy (Jeff Orlinsky, Wheel DynamiX, Natick, MA) and had him wrap the rest of the car.
Compared to the estimated cost of repairs, I've saved a ton of money using Xpel. Say about $5K to wrap the entire car, plus small payments to peel various panels and replace with fresh Xpel after another mis-adventure, versus easily $8K-$10K in auto body work over a period of time. Peel-And-Replace is also very fast, usually an hour or two -- another major benefit.
Over the full 3 years I've had the car, I'd rate the Xpel as one of the best "investments" I made to protect the vehicle and lessen repair costs. When I buy my next S, perhaps in only a couple of months, I'll definitely do another full Xpel wrap. I may not bother with the Opticoat underneath, though, I'm not sure it's that valuable if you're going to wrap the entire car.
For a Model S starting at $65K-$75K, I'd recommend the wrap (full or partial).
For a top-end Model 3, I think I'd still recommend it.
For a model 3 at $35K.... well... I'd bet the full wrap is cheaper than an S because it's a smaller car, but I don't know how much cheaper. I'd probably still wrap it, but it might be a harder decision. Also, I might be swayed by whether the 3 turns out to be all-steel or maybe it turns out to have a lot of aluminum. Aluminum is still much more expensive to work with at an auto body shop than steel. The more aluminum in the visible portions of the model 3 body, the more I'd be inclined to pay for a wrap.
I hope you love your Model 3 -- I've ordered a couple for my family -- as much I have loved my S... these are great cars!
Alan