It has one on the screen. You can also have it beep the horn when it locks so you know it is locked.
the car can be locked on the screen or on the app.
A lock button on the screen does absolutely no good for locking the car when you leave. And it is a frigging pain in the arse to have to pull out my phone, launch the app, scroll to the appropriate screen, and then press the lock button. Consider what happens with the Prius, a car that does lock/unlock right: You never have to take the fob or anything else out of your pocket. You get out of the car, close the door, and press the button to lock it. Return to the car, and lightly touch the inside of the door handle, and it unlocks.
Now consider the Model 3: You get out of the car, walk away, and about the time you are out of earshot, it locks and makes a quiet beep. If you forgot that your BT is off, it doesn't lock, and you don't know it because half the time you don't hear it lock anyway. Your alternative is to fruckle around with your phone to lock the car, or pull out the key card to lock it.
Today I nearly had a disaster: It was my day for kayaking again. (On canoeing days I walk to the club, but on kayaking days I drive to wherever we've decided to launch.) I got there, and turned off BT because I want to leave the phone in the car rather than taking it out on the water. I grabbed some of my gear and shut the door and carried my handful of gear over to the kayaks.
But with the BT off, the car thought it had gone out of range, and when I shut the door, it locked. I had not grabbed the otter box with the kay card, so now the car was locked with both phone and card inside! (The Prius won't lock with the key fob inside! The Model 3, as I learned, will happily lock you out.) The spare key card was back at home, half an hour's drive away. I was going to lose an hour on the water and have to beg a ride home and back. I could not even go kayaking first because my hat and dark glasses (both essential for spending several hours out on the water, in the tropical sun) were locked in the car.
Then I had a brain wave: And finally I come to one good thing about the phone as key!: I got my friend to download the Tesla app on his phone, logged in to my Tesla account, and told the app to unlock the car. It asked me "Are you sure you want to remotely unlock the car?" I answered Yes, and voila! my car unlocked. Then I had my friend delete the app from his phone. We only lost about 3 minutes, instead of an hour.
(When I had the Prius, I put the fob in the otter box, put that in a dry bag, and brought it with me on the water. But I don't want to risk taking the phone out on the water, though some folks do.)