How to rename the titles of several tracks at once in iTunes 12.3?

Not the names, no, not unless you use an Applescript. I guess the programmers couldn't see an instance where you would wish to give a bunch of tracks the identical name.

I guess the most efficient way to do this would be to use the Get Info window. First, arrange the tracks so they're all listed in a row -- drag them all into a temporary playlist if you need to. Then select the first track and type Command-I (or select File > Get Info from the menu bar).

Now when you type in the new song name, you just hit the Forward arrow at the bottom of the screen, and this will bring up the next track, with the Song field already highlighted, so you just have to start typing. I can't think of anything faster than this.

Unless, as Limnos suggests, you really do want to give all the tracks the same name -- that would be a lot faster.