Will it be possible to mount/install a roof rack on Tesla Model 3?
We don't know since they haven't released details of the final car. But on the S, the pano roof is required for a roof rack.
Possible to have a similar configuration as the S. Possibly.
It would be rather strange if Tesla didn't use the same thread size on the Model 3 that they use on the Model S. Tesla already sells a roof rack, and it's easy enough to adjust the width. They could save a few dollars by leaving off the mounting points, in which case it would be possible to clamp a roof rack onto the frame of the car or strap it the way aftermarket ones work. But considering that the roof will likely be a paid option, including the mount points with one of them would make sense.
It seems like a feature that, once they'd added it their cars, they'd try to put in on everything that comes after if possible. The extreme amount of glass on a Model 3, even a metal-roofed Model 3, might mean that it simply isn't possible. We'll have to wait and see.
On the Model S, it's only with a glass roof that you get the mounting points.
IMO the mounts should be standard. Having to use hack attachment methods is wrong and can be unsafe as they are often sketchy. These and the rumored standard hitch port would be much appreciated and would make the car more capable with very little extra effort by Tesla.