My $0.02 on some of the previous posts:
Zion: The best/closest Marriott is St George, UT. CY, TPS & FI. Only about an hour from the park. Cedar City FI is good for the Kolob Canyons section of Zion.
Yellowstone: The SHS in Rexburg is considerably closer than the Idaho Falls properties.
Bryce: Rubys Inn is showing it's age. Stay across the street at the BW Bryce Canyon Grand Hotel. Only a few years old and they have a great breakdast buffet.
If you want to visit Bryce & Zion & the North Rim of the Grand Canyon, IMHO the BW in Mt Carmel Junction best fits the bill. Cheap (my last stay there was $61.00/night) and on a golf course.
JRS