I third Henry's and shrink's suggestion. When I only had the Roadster, I charged it with the Roadster UMC off a disused NEMA 14-30 dryer outlet (30A outlet, from which the UMC made 23A available to the Roadster). But when I added a Model 3 to the garage, I needed another charger. I believe the Wall Chargers can share circuits down to 16A, but I splurged and got a 100A circuit in the garage to share between two new wall chargers, one of which uses a CAN-SR to connect to the Roadster.
The two chargers intelligently negotiate how to apportion the 80A that are available from the 100A circuit. They do a good job of it, too. If both cars need a charge, they'll offer 40A to each one, but as one of the cars begins to draw fewer amps because it's approaching full (it's usually the Roadster) the wall chargers will offer more amps to the Model 3, up to its 48A limit. But because I only drive one car at a time, one of the cars is usually in the garage already full when I arrive with the other car. In those cases, the non-full car gets the full 80A offer.
It works exactly as it should. And as has been mentioned, you can configure up to four wall chargers in this way. For my situation --one driver with two cars, a 100A circuit was absolutely overkill because I've never needed to charge the Roadster at its full 70A speed, and I'm almost never charging both cars at once. So give some thought to how you expect your three cars to be used, since that will dictate if the 40A from your existing NEMA 14-50 receptacle is sufficient or not. If all three cars are going to used for long commutes daily, you might want to consider a 100A circuit so that each car can charge at 80/3 amps. But if usage will be light, or limited to one or two vehicles per day, sharing 40A might be plenty.
