The Siri feature, spotted by Reddit users and Electrek, has been available on the Model S and Model X since November of last year. It’s arguably more useful on the Model 3, though, since that EV is much more tech dependent. To be clear, however, the Tesla app commands controlled by Siri aren’t useful while you’re driving.
For that, you need the built-in voice control, and as it stands now, you can only use that for navigation and calling. However, Elon Musk recently tweeted that Tesla plans to add vehicle control functionality to the built-in voice systems. That might let you set the cruise control and do other things that are currently fiddly to do on the touchscreen. Don’t expect Tesla to tackle that until it gets Model 3 production where it wants, though.