Keep in mind that average temps for the area are pretty much usless in a very hot climate. As St George some years in August can be anywhere between 100 and 115 or so degrees F. More likely on the higher side of that scale. August is very hot. I would suggest visiting in early Nov, weather is usually quite mild and nice in comparison.
Zion and Bryce are at higher elevations, but early Nov is still usually good weather for Zion.
Late April early May, or very late Sept/Oct are usually the best months to visit these areas in general weatherwise, but are also the most popular times. If you are thinking of booking somewhere during those time periods or you want to stick to August, you would need to make those reservations for Hotels ASAP.
When its hotter weather, most kinds of hiking ect must be done in the early morning hours, which Imo doesnt leave you much to do for the rest of the day. I would not even consider visiting Zion in August as its just not comfortable weather Imo.
Keep in mind, that Zion is about a 45 min drive or so from St George, so if your plans include mostly visiting there, you may want to stay closer to that area, such as Springdale which is right near the entrance to Zion.
If you would like help finding a place to stay in St George, you can give us a budget per night, and what kind of accomodations you are looking for. There are also a few good restaurants in the area, as well as lots to do just in the St George and area surrounding.