Be happy that “only” Serial3 is broken. With a bit of luck, you might use these pins still as GPIO pins (check with an oscilloscope or a multimeter). Shorts to ground can theoretically kill the whole Teensy, thus you had still luck. But to prevent unwanted long term effects, I’d considering ordering a new one, or better two or three. It is always good to have a few spares in the drawer in case such “ooops” situations happen.