Serial to Ethernet Connector is licensed according to the number of installations. The basic license option is called Two-License Pack. It allows installing and activating the product on two systems.
As you correctly assumed, in your scenario you will need 2 Two-License Packs. Since this set will cover 4 systems in total, while you need only 3, you will have one spare license which, of course, you'll be able to use anytime in future.