It seems some people have been complaining about water draining into the trunk. The vendors in China have been listening and now you can buy a diverter that looks like a tiny spoiler. Ebay seems to be the cheapest at US$37 while Aliexpress vendors are charging up to US$90.
For Tesla Model 3 18-19 Gloss Black Trunk Spoiler Cover Water Retaining Wing | eBay
For Tesla Model 3 18-19 Gloss Black Trunk Spoiler Cover Water Retaining Wing | eBay