“Nobody at Tesla has ever seen rain before.” (twitter.com)

The s has a lip around the door so rainfall doesn't go into the trunk. It seems stupid they dropped that on the 3.

I have a Model 3. Not a serious problem. I have seen a few drops of water fall into the trunk once, but only when parked on a slope. If the car is relatively flat the water will fall to the side. Also, you can access the trunk from the back seat if you really don’t want to get water inside.

My Cadillac has a similar issue. It's also a known problem with certain volkswagens: https://www.youtube.com/watch?v=bkSuyXnCSO4