Hah, I had a power outage over the summer, at the peak of summer heat actually, for three straight days, while my plants were at 30 days of flowering. I couldn't really do a lot, so they suffered through 99.5 degree heat during the days, with virtually no ventilation the whole three days. For light, what I did was get some flashlights and LED flashlights, and turned those on during what would normally have been their light period. I'm sure it wasn't enough light, not even close, but I think it was just barely enough light for them to stay 'on schedule' until the power was restored. And amazingly enough, they didn't hermie. (But they were spectacularly healthy before the power outage, which probably helped them survive without freaking out.)

They did take some damage, from the heat and lack of ventilation mainly, brown spots on their leaves the rest of the way, but they finished flowering fine.

The three days of darkness thing, at least from what I've read, is something the Dutch experimented with and found that 72 hours of total darkness at the end of FLOWERING can result in as much as a 30 percent increase in THC in some strains, not all. I've never heard of giving the plants a big dose of darkness like that between veg and flowering though.

Either way, do all you can to keep them on whatever their normal schedule is, even if you have to go down Desperation Road like I did, and use flashlights and LEDs for their light periods.