The Thunderbolt 3 port is also a USB 3.1 gen 2 USB Type-C port (well, it is a USB Type-C port supporting USB 3.1 gen 2, and has Thunderbolt 3 as one of the supported USB-C alternate modes). You need a USB-A female (receptacle) to USB-C male (plug) adapter, which can connect your printer’s existing USB cable to your computer. The adapter should support USB 3.1 so it can be used with all USB devices, not just USB 2.0.
Or you can replace the printer’s existing USB cable with a USB-B male to USB-C male cable. Check the printer’s end of the cable to see if it is USB 3.0 or USB 2.0. That will determine which kind of USB-B male connector you should get. I believe a USB 2.0 Type-B plug will connect to a USB 3.0 Type-B plug, but it will not support USB 3.0 speeds.
If your printer has an ethernet or wifi connection ability (to allow connecting it to a network), then you should use that instead, so that you don’t need to connect it directly to your computer. Your computer will be able to find the printer on your local area network (LAN). You should have a router in your house. Your cable modem or whatever device you use to connect to the internet (WAN or wide area network) might have a built in router or maybe built in wifi router. Otherwise, you can buy a router which connects to your internet device on the WAN side, then all your computers, tablets, printers, etc connect to the router on the LAN side. This way, all the devices can use the printer.
If the printer is connected to your computer instead of the network, then your computer can share the printer so other computers can use the printer, but your computer needs to be on to do that. That’s why a network connection for the printer is preferable.
