From an historical standpoint, Tesla has gone from offering 40, 60, 70, 85, 90 kwh battery options and now has simplified that down to only 70D and 90D kwh. So, I don't think they will offer a 55, 60, 65 option because that might be going backwards a bit. I think they might keep it simple and just offer a 70kwh battery pack for the Model 3. They will probably be able to reduce the price of the pack to be reasonable for the Model 3 with economies of scale and maybe a new battery architecture. Easier to max production since all 3 cars will utilize same battery sizes.
But also to go against my own point, Model 3 is smaller and will be on a new platform, so maybe they will go with the smaller 60kwh battery .
Its fun to speculate.