Here's my take on these questions. I recently test drove 2021 MYLR, 2021 M3LR, 2021 M3P, and several other non-Tesla family-friendly vehicles both EV and ICE. We eventually settled on M3P. It's a second car for us, to complement our 2013 S P85 which was our only car for many years.
(I'm skipping the outlet questions as I don't have anything to add there.)
1. For those who have driven the Y and 3, how much bigger is the Y rear seat? I know it has a lot more storage but I think the 3 will work trunk wise. I also know the Y has a bigger back seat but my question is HOW MUCH of a differentiator is this in your experience. Is the 3 comfortable enough (Rear seat wise) that while it's smaller it's just fine or do you find yourself wishing you had gone with something bigger?
The Y back seat is MUCH more comfortable to me as a taller person. The 3 back seat is very low to the floor. There is enough legroom in the 3, but my knees are in my face and it would be pretty horrible for a long trip.
This is a common problem with EV sedans on a "skateboard" battery layout. That layout takes up vertical room, so either you end up with limited vertical space (Model S, Model 3, Lucid Air, etc) or you end up building a crossover just to get decent vertical height (Model X, Model Y, Mach-E, ID.4, etc). The one exception I've driven and sat in is the Polestar 2. It has its batteries in a big center tunnel, not under you feet. So the center tunnel intrudes, BUT you get real vertical height while still feeling like a sedan (not a crossover!) from behind the wheel. Less rear legroom than Model 3, but just enough for me to sit behind myself, so overall much more comfortable back seat to me. It's a pretty great car btw, drives fantastic with the Performance Pack suspension, and it's a hatchback/fastback, but compared to Model 3 it has less range and no supercharging. That made all the difference for us.
HOWEVER...the Model 3 is just way, way nicer to drive than the Model Y! In my opinion, at least. =) Yes the cars are very closely related, and they do feel
similar to drive...but somehow the 3 feels much lighter on its feet, more nimble, better ride quality (not great but better than Y), less wind noise...the experience just comes together much better in the 3. The difference is greater than I expected from just the curb weight difference. The Y does
not drive like a hatchback 3, unfortunately.
I love the 5 seat Y's packaging, its back seat is great, visible is excellent, cargo hauling is solid, it's a very good car...but I honestly couldn't stand how it drove. What it gains in back seat and cargo hauling, it sadly loses in driving experience. Just my opinion! We tested the Y first and I outright rejected it due to ride quality + wind noise. A week or two later Tesla finally had the 3 available to test again, and within minutes I realized I like driving the 3 so much more.
3. Anyone doing mainly country driving? I have winding country roads and about a 15 mile commute. Wondering how much of that 350 range I'll realize at 30 miles a day or will it be closer to some other number (expecting less but HOPING it's not substantially less with this sort of driving).
We do lots of twisty back road country driving! But we're waiting for our new M3P's PPF appointment before taking it on any long drives. I've also ordered smaller, lighter wheels (20" -> 18") which may help efficiency. Next month I'll start having some data on its efficiency. Obviously it's extremely dependent on driving style, wheels+tires, climate, exact roads, etc. But something is better than nothing
One thing I want to mention, if you're into driving fast on twisty back roads: The stock Model 3 suspension (M3LR or M3P doesn't really matter) is...junk, honestly. It feels sporty when you're driving casually, because you feel the road texture (not too busy unlike the Y) and the steering ratio is very quick (which I like!). And on smooth wide turns, like highway onramps, it can grip very well with summer performance tires.
But when you really start pushing it hard on uneven roads with lots of tight turns, bumps, dips, poor pavement, etc...it all falls apart, the shocks become completely overwhelmed. The weight keeps bouncing around, never settles down. The back crashes into the bump stops if the dip is big. The steering feels slow to react. Etc. With a smaller, nimble-feeling car like the 3 I love that kind of driving, so the day after we ordered our M3P, I ordered Öhlins DFV dampers (coilovers) for it. The Polestar 2 I tested had Öhlins DFV dampers from the factory, and the damping was fantastic. You could tell it was very firm, yet ride quality was really good, small stuff was completely smoothed over and big stuff never upset the car. The harder I pushed that car with the Öhlins the better it felt, super quick steering reactions (despite slower steering ratio), and always stable and in control.
If you're not the kind of driver to really push your car hard on back roads, you won't notice these issues with the stock Model 3 dampers, they're fine for casual driving. Since you mentioned country driving though, if your country roads are as twisty and fun as mine, I figure I should mention this.
Zero regrets about our M3P so far. Like any car there's plenty I can nitpick, no car is perfect, but I love driving our M3P even around town, it's so much more fun than a bigger car, but still just big enough to be a 2nd family car for us. I am glad my wife gave me permission to modify the suspension though, otherwise I probably would regret not getting the Polestar 2 Performance.
Also, since we're 7+ years into being EV-only family...zero regrets about that too! We considered and test drove some ICE cars in our recent car shopping, and realized that if we got an ICE neither of us wanted to be stuck with it as a daily driver. We just couldn't go back.
Now for a weekend fun toy I would certainly pick an old ICE sports car with nice engine noises and a manual transmission, but for a daily driver family car an EV is just a wholly superior experience in my opinion, so long as you can charge at home.