Full Disclosure: I am a long-time automotive noise and vibration engineer and live in the Bay Area working at one of the many EV startups (not Tesla). But I did spend nearly 5 years at Tesla developing the NVH behavior of the Model S (and beginning the Model X), so I know well the noise behaviors of these cars. I am happy to say that I just received my own Model 3 (VIN17xxx) and naturally performed some noise benchmarking tests on it. I have access to a Model S P100D (VIN160XXX) and a Mercedes S-550. Below are results of my road noise testing. I test these cars on a moderately coarse surface at 45 MPH (to minimize wind and engine noise), at both the driver's outboard ear and right rear passenger outboard ear locations.
Model 3 - 67 dBA
Model S - 65 dBA
MB S-550 - 61 dBA
Model 3 - 69 dBA
Model S - 67 dBA
MB S-550 - 62 dBA
As you can see, the Model 3 performs well (in general, anything lower than 70 dBA on this road is good), and while not quite as quiet as the Model S, it nonetheless is very comfortable. Of course, the MB S-550 is the champion here and nothing I have ever tested comes close. You'll also notice that the rear seat is consistently louder than the front, but that the S-550 does a better job of balancing that. The Model 3 rear seat does sound a little "open" due to noise coming from the trunk area, but it is not objectionable.
Kudos to the Tesla NVH Team as this is very good performance for a car in this class. I will also mention that wind noise is actually better in my Model 3 than in the Model S I have access to. The door sealing system (especially near the mirror base) is much improved in the Model 3 and it makes a huge difference. Nice job!