Of course, Tesla's own timeline for the Gigafactory show that it will not actually be "fully operational" until 2020. That's the year Tesla has said the first such plant will be churning out 50 GWh of annual battery production, to be used in Tesla EVs and the company's Powerwall home energy storage systems. Apparently, whatever production rate the Gigafactory will be able to handle in 2017 will be enough to get the first batch of Model 3s out the door.
So far, all that we've potentially seen of what the Model 3 looks like is (maybe) in this faraway screen shot from the Australian 60 Minutes program. While some have pointed to Tesla's inability to stick to its own timelines as a reason to expect a delay in the Model 3 launch, Tesla remains steadfast that production will start in 2017. Musk's Tweet reinforces that schedule.