In MPGe terms, any combined rating under 100 would be disappointing in my view. Realistically, I think it's going to come in between 105-110 combined, with only a small differential between the city/highway ratings.
A combined rating in the 110-120 range would be a welcome accomplishment, as that is the range currently occupied by the Nissan LEAF (112/114) and e-Golf (116), each of which has a 24-30kWh pack, weighs about 3400 pounds and is tuned more for efficiency than performance. The BMW i3 (non-REx) has a performance profile much closer to what I'd expect out of a Model 3, but its top-notch 124 MPGe rating owes a lot to its tiny curb weight (~2700 pounds). That's going to be a hard number to match or beat.
If a Model S60 weighs about 4500 pounds, I'd expect a Model 3-60 to weigh about 3900 pounds, due to the ~20% reduction in size and partial replacement of aluminum with steel. At that weight, with much better Cd than the EVs listed above and a slight performance bias, a combined rating just under 110 seems about right, per my fuzzy math.