I wouldn't worry too much. Twenty-one days is the "normal" time for chicks to hatch, but they can hatch earlier or later. I've had chicks hatch as late as 23 days, and as early as 19 days. Too low of temperatures can cause later hatches, but if it was at 99, that shouldn't be the reason (unless the therometer was inaccurate). Some chicks just hatch later than others.
Its possible that the three eggs have died, but not likely. I've only had a few late deaths. If you heard them peeping the previous night, they should still be all right.
Give the eggs at least another day before worrying too much. I've noticed that my eggs often pip at night-- you may wake up tommorow and find that you have three new chicks!
Good luck with the hatch!