In January of 2015, Elon confirmed that the Tesla Model 3 will be sold starting at $35,000 before any tax rebates or incentives (source: jalopnik). First production of the Model 3 is reportedly beginning in 2017 (probably the end of 2017 if I had to guess).
Tesla's website does a great job summarizing the tax incentives that are available currently for their Model S and X customers. In the US right now, current Tesla purchases are eligible for a $7,500 federal income tax credit, and individual states offer their own benefits. Colorado residents are eligible for an additional $6,000 tax credit, lucky them!
The question remains, however, exactly what incentives will be around by the time customers can actually purchase their Model 3. It doesn't matter when you order the car, any possible tax incentives only come into affect once the car is put into service. Assuming the tax credit for electric cars remains as it is now, the federal $7,500 credit begins a phase out once Tesla sells 200,000 cars to U.S. customers. It is very important to understand that the credit is not all or nothing, there is a phase out that once Tesla crosses that 200,000 mark, there will still be some credit available for the following year of sales.
In February 2016, dhanson865 over at TeslaMotorsClub.com has a great post where he breaks down his take on the federal tax incentive phase out schedule.
According to this same post, as of February 2016, Tesla has sold just under 67,000 cars to US customers (out of the 200,000 incentives available, and about 26,000 of those cars were sold in calendar year 2015). Below is some more great analysis from dhanson865 on TMC about the tax incentive phase out timing.
Since this TMC post, Elon confirmed there will be no 'Founders' edition of the Model 3, but he was quoted as saying that higher spec'd Model 3 orders would be produced first (high profit margin). It will be interesting when pre order/reservation holders are finally able to spec out their cars how all of that is handled as you will have to juggle possibly receiving the car quicker if you order more options (and thus maybe will get more of a tax rebate).
There's also a credit available for tax year 2016 for 30% up to $1000 for consumers and 30% up to $30,000 for businesses for charging station cost and install (Tesla sells a $750 high powered wall connector - HPWC - that seemingly would qualify for the credit). Most home customers install a 240 volt NEMA 14-50 outlet which isn't clear to me if it's covered with this credit, so best to consult a tax professional. Also, some Tesla owners report problems claiming this credit in the same year as their $7,500 tax credit, so if you're sure you're buying a Tesla Model 3, it may pay to apply for this credit in 2016 in preparation for receiving the car in 2017/2018 (and hopefully you don't move). Here's the 2015 IRS form for reference & here's a great discussion over at TMC on the topic.
Finally, not a tax incentive, but some electric companies may offer additional incentives for customers that have an electric vehicle, so that's another bonus that may be worth looking into. For example, our local provider PECO will give you $50 per car for letting them know you have an electric car.