As it so happens, my SPIFFS became corrupted yesterday which was causing me weird script corruptions... and the only way to get back on an even keel was to do a reformat to reset everything (it is now ok).
The V3 Format option works noticably differently to V2.
You MUST run the Flasher.exe AS ADMINISTRATOR or the Format option doesn't work.
It doesn't sequentially rattle through all addresses as it does in V2, it quickly returns some sort of ok confirmation in a couple of secs.
Don't do anything else to it for about 15 secs after that though, because my own obsevation is that the blue led gives a brief wink after about 12 secs, and I wouldn't want to interrupt anything it might be doing during that time.
The other option of course is to click the Format button from the browser config page.
The only way you'll know the online format is doing anything is because the browser should lose connection with the esp, which should have returned back to defaults.