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Interior Temperature ?

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From your profile photo it looks like your Y is a lighter color so absorbing less heat. I can't tell if you have aftermarket tint on your 3, but if not then the Y would also have tinted rear windows reducing heat gain through those pieces of glass compared to the 3.

I assume this is only in daytime / sunlight? If they're 5 to 10 degrees off when parked at night or in a garage then there's an issue with one or both of the temp sensors.
 
Is the garage attached to your home? In my attached garage the side closest to the house is definitely warmer, even the cold tire pressure can vary by 1 PSI between the left (furthest) and right (closest) sides of the vehicle.
 
Yes, the 3 is closer to the house.
You could place a thermometer on the dashboard inside each vehicle, one vehicle at a time. Compare the thermometer reading in the vehicle to the Tesla app initial reading when turning on the climate control for that vehicle. This will determine if either vehicle is reading high or low for the at rest temperature inside the garage.
 
Couple of possibilities, temp sensor is off which can be verified with a thermometer or battery heating/placement/configuration allows it to dissipate heat into the 3 more than the Y. I think the bigger question is what temp is it inside the garage? I know my Y pretty much mimics the temp inside the garage, Just looked at it now and the Y says 15c in the car and the garage thermostat says 16c, I take the difference as thermometer accuracy.