Hi Deborah, Thank you for reaching out to us regarding whether the three-leaf sumac (Rhus trilobata) is reasonably fire-resistant.   Unfortunately, if it is not in our list of fire-wise resistant plant list, we are not allowed to recommend it can be even reasonably fire-resistant.    However, I can give you some background info regarding fire wise plant characteristic and you can decide.  Fact Sheet 6.305.
FireWise Plant Materials - 6.305 - Extension (colostate.edu)   

The rhus trilobata (three leaf sumac) is a deciduous shrub and grow 5 to 6 feet tall, and 6 feet wide.  Fact Sheet 6.3.05 states "Deciduous plants tend to be more fire resistant because their leaves have higher moisture content and their basic chemistry is less flammable. Also, when deciduous trees are dormant, there is less fuel to carry fire through their canopies."

The rhus trilobata is drought resistant and has smaller leaves.  The fact sheet states "In some cases, there is a strong correlation between drought tolerance and fire resistance. For example, a plant may shed its leaves or needles during extreme drought. Other drought-tolerant species may have smaller leaves or thick, succulent leaves. These plants offer less fuel or have a higher moisture content, both of which help reduce fire hazard."  But do remove the leaves that fall to the ground in the fall

Also, the location of the shrub is important. The fact sheet states "where and how you plant may be more important than what you plant."   

Where: This article FIRE2012_1_DspaceQuickGuide.pdf (colostate.edu)   talks about where plants such as shrubs and trees should be placed. 

How:  A couple of tips from Fact Sheet 6.305
"Mulch to conserve moisture and reduce weed growth. Mulch can be organic (wood chips or small bark pieces) or inorganic (gravel or rock). Avoid pine bark, thick layers of pine needles or other materials that can easily carry fire."

"Timely pruning is critical. It not only reduces fuel volume but also maintains healthier plants with more succulent, vigorous growth."

Hope you find the info helpful in your decision whether to plant the three-leaf sumac.

-- Colorado Master Gardener