Use these instructions:
Model 3 Automatic Garage Opener - HomeLink
After using the remote, you may have to go to the actual garage door opener unit (the one on the ceiling) and click the LEARN button on it within a few seconds after programming the Model 3, and then press the HomeLink button again on the model 3 one/two more times so the garage door opener "LEARNs" the Model 3 opener.
My opener is old and that had to be done.
Also I suggest that you do the programming with the car in the garage. The Model 3 is using GPS location for where it thinks home is. The tech had done it with the car outside and the car thought this was it's parking location so the garage door would open/close when I was too far from home. This was a pain especially when driving away, because I had not way to knowing it actually shut. and a pain when coming home as I did not want to shut the door (instead of opening) if kids were playing in driveway. Now it happens just as I leave/enter driveway, which I think is way safer in all scenarios.