If you order right now, there's a chance you'll still get the car by the end of the year (which is all that really matters with the credit), but you'd be cutting it pretty close. All Tesla claimed was that Oct 15 is the deadline for guaranteeing getting the car before the end of the year. Likely, there'll be a bit of leeway that will see people ordering within the next few days also get the car by the end of the year, especially with a 4-6 week promised turnaround for the AWD models (6-10 on the RWD one talked about here cuts it really close).

The full $7500 tax credit is applicable on any new, inventory, or demo vehicle previously not registered to someone until December 31st, 2018. You might have to order a Model 3 by a certain date to take delivery before then, but the tax credit is applicable until the end of the year.

Source?