its quite possible. especially with a glass roof even more so if its a black car. It does not mean its 28c everywhere in the car just where the sensor is. Thermometers are cheap If you are concerned its in accurate then get one and compare.
In a previous car I had glass roof. Never remember noticing heat radiating from it like some Tesla owners say, although full sun was sometimes a pita - it has a blind that I could pull over that worked well. Will have a think about the Tesla/after market ones once we have experienced it, but they apparently do a good job of stopping the glass act like a radiator.
There were a couple of times I parked outside on the sunny side of the street on particularly warm days this summer (seems such a long time ago...) and it reached the mid 40s inside the car while it was high 20s outside. Didn't worry about it too much though because, as other say, of the greenhouse effect. Just had to remember to put the climate control on a few mins before getting in. There is also a cabin overheat setting although that is grayed out for me (a quick search suggests some people had this problem because their car wasn't marked as "delivered" - not going to worry about it just now though because it'll be nearly a year before we get the next couple of days of summer).
I am not sure about Model 3, but for Model S the temperature sensor sits in the centre console (below the armrest, next to the USB sockets), with a little fan to pull air through the vent holes so that it is measuring the air temperature rather than the surface temperature of the (black) plastic.
Naturally, when the car is 'off', the fan doesn't run, so if the car has been parked in the sun the sensor and surrounding plastic will be heat-soaked and much higher than the air temperature; although the fan may start up when you poll it from the app, it probably takes some considerable time to settle down to the true temperature.