For that matter, they're all pretty similar to (but smaller than) Q. shumardii & Q. nuttallii too, aren't they?

Aren't Q. nuttallii and Q. texana the same thing? Q. gravesii seems to have narrower leaves than the others...

Yes. Q. texana is Nuttall oak now.

Q. buckleyi occur on limestone hillsides from about DFW to San Antonio west until you get to the western edge of the Texas Hill Country. At it's western edge Q. buckley gets about 18 inches of rainfall per year. It's northwestern limit is/was zone 7. A few growers in New Mexico an Colorado prefer to collect from these trees for drought tolerance and cold hardiness. Trees are deciduous and turn bright red in the late fall.

Q, gravesii grows on both limestone and igneous soils in some of the the mountains of western Texas and Coahuila, Mexico. The leaves are similar as Q. buckleyi and tend to be a little shinier and more persistent. They color later into December to January with golds and reds.

These make smaller trees to about 35 feet or so. So much better where water is a issue.

Sorry for the late response... and thanks for the very detailed answer. So, in general, Q. gravesii is the smaller of the three?

Stubbled on this post via google search. While out at nearby shopping center that has a REALLY nice collection of trees and landscaping, I noticed this tree in a parking lot island looking quite nice. My initial reaction is it's Q. gravesii. If so, I really need to find a nice 15g size to add to my yard as this looks fantastic and should do well on my western exposure. Anyone know a source in/near San Antonio?

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The below picture is of colored up Q. gravesii from NM for reference
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I doubt the tree in the parking lot is a Q. gravesii. It's probably a oddball Q. buckleyi. I don't think any professional type growers are growing Q. gravesii in Texas. The acorns are usually ready in mid to late November in the Chisos mts. in Texas and northern Mexico.