I’m North America Tesla Service Center don’t offer the installation for Model 3. However if you brought a Model Y and you didn’t choose to add the Tow hitch when you ordering the car. You can buy it after delivery of the car by buying the Tow Package and schedule a Service appointment. After installation the Tech will update your software and that will give you “Tow Mode”
But in E.U I believe that option is available for both the 3 & Y