Your Mac has a name. It shows up on other computers when they are on the same network as your, and it shows up various places on your own machine. It’s nice if the name is something other than “Macintosh (3)” so here’s how you make the name be anything you want.
Just go to the Apple menu, then System Preferences, then Sharing, and rename away. Best to do this when no one is connected to your Mac, of course.
Here’s what it looks like on my Mac: