I'm not sure what J3 is for
@demik had to remind me of this, but J3 is for unusual situations where it may be possible to find the !ENBL2 signal for a nonexistent external/secondary floppy port somewhere on the logic board (or clip onto its pin on the SWIM chip, failing that).
The Mac floppy disk interface is effectively a bus, with the only non-shared signal being !ENBL - there's !ENBL1 for the internal floppy drive and !ENBL2 for the external/secondary floppy drive, and DCD/HD20 is only supported off of !ENBL2. Thusly, if you have a machine like the LC II which is suspected of having DCD/HD20 support in the ROM but has only one floppy drive port, you can connect the TashTwenty Tiny board to the primary floppy drive port to get all the other signals and connect the !ENBL2 signal to J3 from somewhere else, and (depending on the ROM) have DCD/HD20 support.
Bridging J3 connects the !ENBL pin on the microcontroller to the !ENBL pin on the 2x20 header, which is what you want for the overwhelming majority of cases where you're connecting the board to an external/secondary floppy drive port that has !ENBL2 in the connector where it should be. It's fine to bridge the two pins with a piece of wire, but if you want this expandability available to you, it'd be better to solder a two-pin header and stick a jumper on it when not in use.