I faced a similar question a little over a year ago.
You will lose adjustable suspension, center cluster, maybe biohazard filter, about half your cargo space, auto trunk, better and quieter ride, better AP controls (if you are going to enable that). The S is better for longer drives.
You will gain a newer car, with quicker acceleration, sharper handling that is more efficient (so less money AND quicker to charge) and easier to park. The 3 is a better value and commuter.
There are tradeoffs, so it all depends on what YOU want (and how much you drive). Would I do it? Well, I already did. But then, my wife still has a Model X - if this was our only Tesla, I would have kept the S.
is a thread with more details on the differences between the cars.