Sometimes the panels under the body of the Model 3 can come unclipped, causing a visible gap.
If you can see the clip, and the clip is still attached to the underbody panel, line up the clip with the hole and hit it with your palm, or closed palm to snap the clip back into place.
If you can’t see the clip, or the clip is missing, you’ll need to take it into the service center. They may have to remove the panel further to recover the clip that popped out. src