I hesitate to reply as I don't own a Tesla Model 3, but I have read a lot from those who do on the Tesla Motors Club forum.
Realistically, I wouldn't expect the Model 3 to get the full EPA rated range when driving at freeway speeds. As I understand it, the EPA's "highway" testing is done at an average speed of 60 MPH. Most people driving long distances are going to be traveling faster on Interstates and freeways. Faster speeds mean less range; that's just physics, no way around it.
Also, the EPA's tests are done with no climate control, and in winter you're probably going to be running the cabin heater (unless you're a masochist), so that will cut into range even if you leave the car plugged in overnight and pre-heat it before leaving in the morning. If you don't leave it plugged in overnight on a cold (below 40° F) night, then expect to lose a significant amount of range.
Those who aggressively practice hypermiling will of course be able to exceed the EPA rated range, but then they can do that with just about any car. That has a lot more to do with the driver than it does to do with the car.
Just a couple of data points from claims made in
one discussion at the Tesla Motors Club forum:
Two owners estimated the range of the LR Model 3 (currently EPA rated between 310 and 325 miles, depending on trim level), when traveling at 80 MPH in summer with the A/C running, at 250 miles. In winter when traveling at sub-40° weather, I would expect even lower range. Above that temp the car shouldn't have to waste energy heating the battery pack, so hopefully it wouldn't be
much worse than running the A/C, altho certainly Tesla's resistive heaters do draw more power than the A/C.
Heat pumps used in some of the newer cars have a significant impact on EV range, when used for cabin heating. If I recall correctly, the Kona benefits from that; this is one of the few areas where Tesla's tech actually lags behind some of its competitors.
Unfortunately, Wildeyed, no one can give you a "one size fits all" answer. It depends on your driving style, on whether or not the road is hilly or has significant elevation changes, on the temperature (especially in winter driving), and on other weather conditions. Even rain will reduce the range a bit. The EPA range should be considered optimal range when driving at 60 MPH in fair weather on flat roads. For most people, real-world range at freeway speeds will be noticeably less.