If you have the 2019 Model 3, the underneath cover has an opening for the hitch, otherwise you will need to make a cut.
Which hitch did you ordered? The best are the one that can be hidden, I mean that you can easily remove the tongue when not in use.
This can be preferable to avoid touching the ground like when entering into a garage driveway with a ramp.
I have seen video on you tube where someone made a cut into the plastic cover bumper, so the hitch is flush with the under body of the car.
This might be necessary if you have the performance, which is lower.
Do you have a 1"1/4 or a 2" hitch? And which bicycle mount are you using?
There are tons of YouTube videos dealing with the installation. Something very important is that the hitch will be made of iron,
while the rear panel is made of aluminium. So during the installation, the parts need to be isolated with some plastic films,
and the bolts need also to be isolated to avoid corrosion.
Note: Did you took advantage of a President Day sales?